Darth Tenebrous is an obscure and cryptic Sith Lord but a powerful one, and a favorite of mine. I just wanted to convey his powers and abilities. Here is a supplementary case on Tenebrous' powers.

Here is another respect thread for Tenebrous, with more detail and scaling.

Respect Darth Tenebrous:

Power

Each Banite Sith Lord grew more powerful than the last. After approximately a thousand years and thirty or so Sith Lords in the Banite line, Tenebrous served as one of the final Sith Lords in the Order of the Sith Lords, thus making him more powerful than any of his predecessors and vastly more powerful than the likes of Darth Bane, Darth Zannah and Darth Vectivus:

For a millennium, the Sith maintained the order in secrecy, passing down their evil heritage. As they gained knowledge of the dark side of the Force, their powers increased with each generation.

Source: Episode 1: The Phantom Menace Scrapbook

Ultimately, Bane's plan produced more powerful Sith Lords with every generation.

Source: Force & Destiny

For a thousand years we continued to follow Bane's Rule of Two, existing in the shadows, biding our time, growing in power, feeding our hatred.

Source: Insider #88: Heritage of the Sith

"Bane's power has been passed down for a thousand years. I vow to be its last recipient."

Source: Book of Sith: Secrets from the Dark Side

Force Speed

Tenebrous bolts so quickly that Plagueis is nearly bowled over, and struggles to keep pace with him. By this time, Plagueis is capable of appearing as a blur to droids who have no difficulty avoiding blaster fire, and can run/fight so quickly that he would have appeared to an onlooker as lightning racing throughout the forest understory:

Nearly knocked over by the swiftness of Tenebrous’s departure, Plagueis had to call deeply on the Force merely to keep up.

Source: Darth Plagueis

Plagueis recalls that he and Tenebrous had fought an army of Kursid warriors wielding bows and lances. Judging from Plagueis and Sidious's later battle with the Kursid warriors, Plagueis and Tenebrous would have only used force pikes rather than lightsabers, disused their offensive Force powers, and exercised nothing except speed and agility and still emerged unscathed after defeating the Kursid warriors:

The location of the planet known to the Sith as Kursid had been expunged from the Republic records in distant times, and for the past six hundred years had been reserved for use as a place of spectacle. Masters and apprentices of the Bane lineage had visited with enough regularity that a cult had come into being in that part of the world based on the periodic return of the sky visitors. The Sith hadn't bothered to investigate what Kursid's indigenous humanoids thought about the visits—whether in their belief systems the Sith were regarded as the equivalent of deities or demons—since it was unlikely that the primitives had yet so much as named their world. However, visiting as apprentice and—more often than not—as Master, each Sith Lord had remarked on the slow advancement of Kursid's civilization. How, on the early visits, the primitives had defended themselves with wooden war clubs and smooth rocks hurled from slings. Two hundred years later, many of the small settlements had grown to become cities or ceremonial centers built of a crude sort, and magical guardian symbols had been emblazoned on the sloping sides of defensive walls. At some point previous to Darth Tenebrous's visit as an apprentice, replicas of the Sith ships had been constructed in the center of the arid plateau that served as a battleground, and enormous totemic figures—visible only from above—had been outlined by removing tens of thousands of fists-sized volcanic stones that covered the ground. On Plagueis's first visit, some fifty years earlier, the warriors he and Tenebrous had faced had been armed with longbows and metal-tipped lances.

That the Sith had never demanded anything other than battle hadn't kept the primitives from attempting to adopt a policy of appeasement, leaving at the ships' perpetual landing site foodstuffs, sacrificial victims, and works of what they considered art, forged of materials they held precious or sacred. But the Sith had simply ignored the offerings, waiting instead on the stony plain for the primitives to deploy their warriors, as the primitives did now with Plagueis and Sidious waiting. Announcing their arrival with low runs over the city, they had set the ship down and waited for six days, while the mournful calls of breath-driven horns had disturbed the dry silences, and groups of primitives had flocked in to gather on the hillsides that overlooked the battleground.

"Do you recall what Darth Bane said regarding the killing of innocents?" Plagueis had asked.

"Our mission," Sidious paraphrased, "is not to bring death on all those unfit to live. All we do must serve our true purpose—the preservation of our Order and the survival of the Sith. We must work to grow our power, and to accomplish that we will need to interact with individuals of many species across many worlds. Eventually word of our existence will reach the ears of the Jedi."

To refrain from senseless killing, they wielded force pikes rather than lightsabers. Meter-long melee weapons used by the Echani and carried by the Senate Guard, the pikes were equipped with stun-module tips capable of delivering a shock that could overwhelm the nervous systems of most sentients, without causing permanent damage.

"The next few hours will test the limits of your agility, speed, and accuracy," Plagueis said, as several hundred of the biggest, bravest, and most skilled warriors—their bodies daubed in pigments derived from plants, clay, and soil—began to separate themselves from the crowds. "But this is more than some simple exercise in our rise to ultimate power, and therefore servants of the dark side of the Force. Centuries from now, advanced by the Sith, they might confront us with projectile weapons or energy beams. But then we will have evolved, as well, perhaps past the need for this rite, and we will come instead to honor rather than engage them in battle. Through power we gain victory, and through victory our chains are broken. But power is only a means to an end."

To the clamorous beating of drums and the wailing of the onlookers, the warriors brandished their weapons, raised a deafening war cry, and attacked. A nod from Plagueis, and the two Sith sped across the plain to meet them, flying among them like wraiths, evading arrows, gleaming spear tips, and blows from battle-axes, going one against one, two, or three, but felling opponent after opponent with taps from the force pikes, until among the hundreds of jerking, twitching bodies sprawled on the rough ground, only one was left standing. That was when Plagueis tossed aside the stun pike and ignited his crimson blade, and a collective lament rose from the crowds on the hillsides.

Source: Darth Plagueis

Combat Skill

Tenebrous recalls killing his master with "customary efficiency":

He had exterminated his doddering Master with his customary efficiency, and had embarked immediately on a decades-spanning quest for an apprentice of his own.

Source: The Tenebrous Way

Tenebrous makes Venamis a master in Plagueis's fighting style, enabling him to anticipate and counter Plagueis's every move. Venamis also manages to twist his body in surprisingly limber fashion, fight while suspended in the air, and utilize multiple lightsaber forms ambidextrously. As Tenebrous was Venamis's personal tutor in lightsaber combat and Venamis was in awe of Tenebrous, it would only be plausible to suggest that Tenebrous could accomplish all of this as well:

For no sooner had the blades of their weapons clashed than Venamis began to bring the fight to him in unexpected ways, twirling his surprisingly limber body, tossing the lightsaber from hand to hand, mixing forms. At one point he leapt onto an overhanging greel branch and, when Plagueis severed it with a Force blow, hung suspended in the air—no mean feat in itself—and continued the fight, as if from high ground. Worse for Plagueis, Tenebrous had made Venamis an expert in Plagueis’s style, and so the Bith could not only anticipate but counter Plagueis’s every move.

Source: Darth Plagueis

Tenebrous would have had to have mastery over exotic weapons in order to utilize only Force pikes and defeat an entire army alongside Plagueis:

The location of the planet known to the Sith as Kursid had been expunged from the Republic records in distant times, and for the past six hundred years had been reserved for use as a place of spectacle. Masters and apprentices of the Bane lineage had visited with enough regularity that a cult had come into being in that part of the world based on the periodic return of the sky visitors. The Sith hadn't bothered to investigate what Kursid's indigenous humanoids thought about the visits—whether in their belief systems the Sith were regarded as the equivalent of deities or demons—since it was unlikely that the primitives had yet so much as named their world. However, visiting as apprentice and—more often than not—as Master, each Sith Lord had remarked on the slow advancement of Kursid's civilization. How, on the early visits, the primitives had defended themselves with wooden war clubs and smooth rocks hurled from slings. Two hundred years later, many of the small settlements had grown to become cities or ceremonial centers built of a crude sort, and magical guardian symbols had been emblazoned on the sloping sides of defensive walls. At some point previous to Darth Tenebrous's visit as an apprentice, replicas of the Sith ships had been constructed in the center of the arid plateau that served as a battleground, and enormous totemic figures—visible only from above—had been outlined by removing tens of thousands of fists-sized volcanic stones that covered the ground. On Plagueis's first visit, some fifty years earlier, the warriors he and Tenebrous had faced had been armed with longbows and metal-tipped lances.

That the Sith had never demanded anything other than battle hadn't kept the primitives from attempting to adopt a policy of appeasement, leaving at the ships' perpetual landing site foodstuffs, sacrificial victims, and works of what they considered art, forged of materials they held precious or sacred. But the Sith had simply ignored the offerings, waiting instead on the stony plain for the primitives to deploy their warriors, as the primitives did now with Plagueis and Sidious waiting. Announcing their arrival with low runs over the city, they had set the ship down and waited for six days, while the mournful calls of breath-driven horns had disturbed the dry silences, and groups of primitives had flocked in to gather on the hillsides that overlooked the battleground.

"Do you recall what Darth Bane said regarding the killing of innocents?" Plagueis had asked.

"Our mission," Sidious paraphrased, "is not to bring death on all those unfit to live. All we do must serve our true purpose—the preservation of our Order and the survival of the Sith. We must work to grow our power, and to accomplish that we will need to interact with individuals of many species across many worlds. Eventually word of our existence will reach the ears of the Jedi."

To refrain from senseless killing, they wielded force pikes rather than lightsabers. Meter-long melee weapons used by the Echani and carried by the Senate Guard, the pikes were equipped with stun-module tips capable of delivering a shock that could overwhelm the nervous systems of most sentients, without causing permanent damage.

"The next few hours will test the limits of your agility, speed, and accuracy," Plagueis said, as several hundred of the biggest, bravest, and most skilled warriors—their bodies daubed in pigments derived from plants, clay, and soil—began to separate themselves from the crowds. "But this is more than some simple exercise in our rise to ultimate power, and therefore servants of the dark side of the Force. Centuries from now, advanced by the Sith, they might confront us with projectile weapons or energy beams. But then we will have evolved, as well, perhaps past the need for this rite, and we will come instead to honor rather than engage them in battle. Through power we gain victory, and through victory our chains are broken. But power is only a means to an end."

To the clamorous beating of drums and the wailing of the onlookers, the warriors brandished their weapons, raised a deafening war cry, and attacked. A nod from Plagueis, and the two Sith sped across the plain to meet them, flying among them like wraiths, evading arrows, gleaming spear tips, and blows from battle-axes, going one against one, two, or three, but felling opponent after opponent with taps from the force pikes, until among the hundreds of jerking, twitching bodies sprawled on the rough ground, only one was left standing. That was when Plagueis tossed aside the stun pike and ignited his crimson blade, and a collective lament rose from the crowds on the hillsides.

Source: Darth Plagueis

Barrier

Tenebrous forges a Force Barrier that repels an explosion potent enough to sunder an entire mine and collapse cave ceilings. By comparison, Plagueis, despite being several metres behind Tenebrous, is knocked to the ground by the sheer force of the blow:

Overhead, hawk-bats took panicked flight from their roosting places in the stippled ceiling. Natural light in the far distance prompted the two Sith to race for the grotto; but, even so, they were a moment late.

The gas explosion caught up with them just as they were entering the light-filled cavity at the top of the escarpment. From deep in the tunnel resounded a squealing electronic wail, and at the same time, almost as if the cave system were gasping for breath, a searing wind tore down from a perforation in the grotto’s arched ceiling through which the ship had entered. A muffled but ground-heaving detonation followed; then a roiling fireball that was the labyrinth’s scorching exhalation. Whirling to the tunnel they had just exited and managing somehow to remain on his feet, Tenebrous conjured a Force shield with his waving arms that met the fireball and contained it, thousands of flaming hawk-bats spiraling within the tumult like windblown embers.

A few meters away Plagueis, hurled face-first to the ground by the intensity of the vaporizing blast, lifted his head in time to see the underside of the domed ceiling begin to shed enormous slabs of rock. Directly below the plummeting slabs sat their starship.

Source: Darth Plagueis

The mine caverns that were destroyed by the explosion were the size of cathedrals:

Closer to the surface the tunnels opened into caverns the size of cathedrals, smoothed and hollowed by rainwater that still surged in certain seasons of Bal’demnic’s long year.

Source: Darth Plagueis

Lightning

Tenebrous fires Force Lightning that leaves behind residual sparks long after the initial discharge of his powers:

Hastening to the rim of the circular shaft, the two Sith removed their gloves and aimed their long-fingered unprotected hands into the inky darkness. Instantly tangles of blue electrical energy discharged from their fingertips, raining into the borehole. Strobing and clawing for the bottom, the vigorous bolts coruscated into the lateral corridor the probe had excavated. Crackling sounds spewed from the opening long after the Sith had harnessed their powers.

Source: Darth Plagueis

Telekinesis

With no evident strain, Tenebrous telekinetically supports layer after layer of enormous slabs of rock that threatened to crush his ship:

A few meters away Plagueis, hurled face-first to the ground by the intensity of the vaporizing blast, lifted his head in time to see the underside of the domed ceiling begin to shed enormous slabs of rock. Directly below the plummeting slabs sat their starship.

“Master!” he said, scrambling to his feet with arms lifted in an attempt to hold the rocks in midair.

His own arms still raised in a Force-summoning posture, Tenebrous swung around to bolster Plagueis’s intent. Behind him, the fireball’s final flames surged from the mouth of the tunnel to lick his back and drive him deeper into the grotto.

The cave continued to spasm underfoot, sending shock waves through the crazed ceiling. Cracks spread like a web from the oculus, triggering collapses throughout the grotto. Plagueis heard a rending sound overhead and watched a fissure zigzag its way across the ceiling, sloughing layer after layer of stone as it followed the grotto’s curved wall.

Now, though, it was Tenebrous who was positioned beneath the fall.

Source: Darth Plagueis

Force Sense

Tenebrous's Force senses are advanced enough to sense each of his midi-chlorians die out as he dies:

His Force-perception was even more acute than the magnifying powers of his enormous eyes; in the Force, he could feel each individual midi-chlorian wink out in turn, a spreading wave of darkness, like stars eclipsed by the silhouette of an approaching ship.

Source: The Tenebrous Way

Tenebrous's "impossibly refined" perceptions detect the brush of Plagueis's mind:

And now his impossibly refined perceptions detected the brush of Plaugueis' mind, as the apprentice probed the vanishing midi-chlorians of his dying master with his own use of the Force, as Tenebrous had known he would.Source: The Tenebrous Way

Tenebrous's Force perceptions increase as his body's physical senses perish. Tenebrous is able to feel the nearby Plagueis and his physical traits, and he can trace the slightest wisp of Plagueis' "clumsy" attempts to record and analyze every detail of his death:

Now that his body's physical senses had altogether perished. Tenebrous found his perception of the Force to be proportionately heightened. With glorious precision, he could trace the slightest wisp of Plagueis' clumsy Force-probing as his apprentice sought to record and analyze every detail of Plagueis's death. He could feel Plagueis himself: crouched nearby, his eyes closed, the long spiderish fingers of one hand stretched forth as though to snatch Tenebrous' disappearing midi-chlorians from mid-air.

Source: The Tenebrous Way

Tenebrous senses that his body has already suffered irreversible brain death, but his consciousness is still functional. No longer bound by the typical constraints of temporal perception, Tenebrous finds that he can perceive every individual nanosecond and simultaneously comprehend entire galactic eons :

Now, dead at last, he could begin to enjoy the fruits of his lifelong labor. In the Force, he could feel that his body had already suffered irreversible brain-death, yet his consciousness remained, fully aware, fully functional, and connected to the Force in a manner more intimate than he had ever believed possible. Freed now of the crude biological processes that mark the passage of time, Tenebrous found he could perceive the measured tick of each individual nanosecond while simultaneously comprehending the entire sweep of galactic eons.

Tenebrous senses that his apprentice is more powerful than he had originally suspected, and he also finds out that Plagueis's foresight powers were also much more developed than Tenebrous had previously believed. This enables him to foresee Plagueis's death at the hands of Darth Sidious:

Full access to his apprentice's Force-perceptions! Delightful. Better than Tenebrous had allowed himself to hope. Hmm-perhaps he should have invested some time in actually training the foolish Muun. Tapping Plagueis' Force powers would be more entertaining if they weren't so stunted from disuse. And yet....

As he continued to explore, Tenebrous gradually became aware of the full range of his apprentice's connection to the Force, which was considerably deeper, broader, and more powerful than Tenebrous had ever suspected. He reflected, with a twinge of uncomfortable premonition, that perhaps Plagueis had been right when he contended that Tenebrous had always underestimated him.

Now Tenebrous touched upon his apprentice's powers of foresight, which were also vastly more developed than Tenebrous had believed. For a moment. Tenebrous found his perception cast far forward in time-to Plagueis' own death at the hands of his apprentice, who was himself visible only as a smear of darkness....

A shadow!

Source: The Tenebrous Way

Telepathy

Tenebrous appears to have telepathically affected Plagueis's mind to some extent, to ensure that Plagueis would be probing his dying Midi-chlorians - and to a larger extent, Tenebrous's mental manipulations may have been the cause of Plagueis's initial obsession with immortality and Midi-chlorians in the first place:

And now his impossibly refined perceptions detected the brush of Plaugueis' mind, as the apprentice probed the vanishing midi-chlorians of his dying master with his own use of the Force, as Tenebrous had known he would. Tenebrous had spent decades making sure that Plagueis would be unable to resist doing exactly that.

Source: The Tenebrous Way

Intelligence & Calculational/Tactical/Analytical Abilities

Tenebrous possesses matchless intellect and incredible computational power which he believes has led him far beyond the witless fantasies of Sith tradition:

More than a century before, when Tenebrous had been but a Sith apprentice himself, the magnificent computational power of his Bith brain had led him far beyond the simplistic Force studies imposed on him by his Master. He had always been far too intelligent to be seduced by the traditional Sith metaphysical twaddle of dark destiny and the witless fantasy of endless war against the equally witless Jedi Order. Soon he had confirmed to his own satisfaction that the dark side of the Force, far from being some malevolent mystic sentience bent on spreading suffering throughout the galaxy, was in truth merely an energy source, and a tool with which he could impose his will upon reality. It was a sort of natural amplifier he could use to multiply the effectiveness of his many useful abilities. None of which was more useful than his matchless intellect.

Source: The Tenebrous Way

Tenebrous is literally capable of predicting the future via mathematical calculations - in other words, he could calculate the future:

Like many Sith before him, he had turned his powers toward knowledge of the future. But unlike any Sith before him he had the enormous brain of his people, which combined sheer brute processing power with a level of analytical precision simply beyond the capacity of any other species. The future was always in motion, and while other Sith struggled to foresee the faintest, least specific hints of what was to come, Tenebrous had no need to see the future.

He could calculate it.

Source: The Tenebrous Way

Tenebrous's calculations enable him to foresee that there will be an end of the Banite Sith/Rule of Two, and the coming of a "shadow" (Darth Sidious) that would darken the galaxy and mark the end of both the Jedi and the Sith as they had previously been known to the universe, and to start the "One Sith" (not Krayt's One Sith). Tenebrous also calculates that he will not be that Sith Lord:

While still merely an apprentice, his analysis had shown him the inevitable end of the Banite Sith and its preposterous Rule of Two. His calculations plainly indicated the coming of a shadow so vast it would darken the galaxy entirely- so vast it would mark the end of both Jedi and Sith as the universe had known them heretofore. The rise of the shadow would be the end of history itself. Tenebrous had not the slightest doubt that the entire galaxy would measure time according to its arrival. Events would be marked by how far they had preceded the shadow, or by how long after it they followed. Though the exact nature of the great shadow remained occult, the remorseless logic of his extrapolation detailed the coming destruction of the Banite system, and the rise of what would become known as the "One Sith." One Sith! The conclusion was so obvious as to require no confirmation: one single Sith Lord would arise of such power that he'd have no need of any apprentice nor fear of the Jedi. He would take and hold the galaxy by his own hand alone. Without an apprentice- or a Jedi Order-to destroy him, the One Sith would rule forever! A heady prospect, with only a single drawback: Tenebrous was not to be that Sith Lord.

Source: The Tenebrous Way

Tenebrous foresees his own death, which his calculations indicate will take place decades before Darth Sidious's rise:

Tenebrous was not to be that Sith Lord. His own death was clearly foretold, entirely inevitable, and it would precede the rise of the shadow by decades. His fate was explicit in the numbers, and numbers do not lie. However-as Tenebrous came eventually to realize over his many years of research, contemplation and calculation-it might be possible for the numbers in question to be, well, deceived....

Source: The Tenebrous Way

Plagueis realizes what Tenebrous foresees, and then kills him, thus making Tenebrous's calculations correct:

And in that instant Plagueis perceived the danger Tenebrous had foreseen earlier: his death.

His death at Plagueis’s hands.

Source: Darth Plagueis

Tenebrous calculates the potential of the Chosen One:

The key, he'd discovered, lay in an obscure legend obliquely referenced in the Journal of the Whills, about a hero fairly typical in most cultures-the sort of promised future savior who appears in the foundational myths of nearly every developed society. What distinguished this particular savior from his run-of-the-mill equivalents was that he, according to four of eleven possible translations, was to be "born of pure Force." After three standard years devoted specifically to exploring all possible permutations of the interpretation, Tenebrous determined that such a birth was indeed possible, at least metaphorically- "born of pure Force" could be read as indicating the creation of a living being through direct manipulation of midi-chlorian processes in an already living being. And further, as Tenebrous discovered with rising excitement, such a being s Force potential might be limited not by its creator's own midi-chlorian count, but instead only by its creator's level of discipline and attention to detail. Indeed, his calculations indicated a range potentially far beyond his own. With proper execution, the "savior" might have a midi-chlorian count as high as fifteen thousand! Perhaps even more. It might be possible to create a being with the greatest Force potential ever recorded!

Source: The Tenebrous Way

Tenebrous predicts that he will use Essence Transfer to ensure that he will possess his way into the Chosen One's time, and then possess the Chosen One himself. This is the only prediction he makes that is actually inaccurate, but it does not seem to be entirely "calculated" as with his other mental calculations:

And-by the application of his own suitably subtle variation of the ancient Sith brute-force essence transfer-Tenebrous could ensure that his own consciousness would be present at the creation of this being, this savior, this Chosen One. And, at the moment of creation-long before the Chosen One could hope to resist- Tenebrous would seize it. Would become it. With this single stroke, decades after his body's death, he would become the most powerful Force-user in the history of the galaxy. It was all there in the numbers. He could not possibly fail.

Source: The Tenebrous Way

Maxi-Chlorians and Essence Transfer

Tenebrous creates a retrovirus that infects midi-chlorians, turning a tiny fraction of his own midi-chlorians into maxi-chlorians that, instead of migrating into the Force, would migrate into Plagueis.

Amidst the billions upon billions of individual midi-chlorian deaths in Tenebrous' cells were a tiny fraction of midi-chlorians that were not dying.

That would not die so long as they inhabited a living host. These especially tenacious midi-chlorians-Tenebrous had privately labeled them with the indeed, Tenebrous had gone to considerable trouble to ensure it would always remain so.

Instead of actually training his doltish apprentice, Tenebrous had flattered Plagueis' mysticism while pricking his insecurities, sending him off on one useless, doomed-to-fail mission after another. In turn, Tenebrous had invested every available second of the freedom this afforded into designing, creating, and deploying the one weapon that Plagueis would never suspect.

Could never suspect. His own prejudices about the Force ensured Plagueis wouldn't believe such a thing was possible.

Tenebrous created a retrovirus that could infect midi-chlorians.

Midi-chlorians were, after all, merely jesting sobriquet maxi-chlorians-had been altered. Improved. It would not be an overstatement, in Tenebrous' opinion, to use the word perfected. These maxi-chlorians would indeed migrate, but not into the Force.

They would migrate into Plagueis.

Source: The Tenebrous Way

Tenebrous encodes his retrovirus with his own consciousness, and releases it into his bloodstream. This infects a number of his midi-chlorians, turning them into maxi-chlorians. This comes at a cost of partial severance from the Force, but Tenebrous chooses to confine this severance to his ability to sense the motion of the future, as he can already calculate the future, hence he does not believe it is necessary to have fallible foresight powers any longer.

To detect this infinitesimal percentage would require the precision of a Bith; it was far beyond his apprentice's limited perceptions-and symbiotic organelles that contribute to the organic processes of the living cells they inhabit. Due to their role in Force interactions, altering them was singularly challenging-they had an unsettling tendency to spontaneously express unexpected and unfortunate side effects-but by applying the full analytic prowess of his vast Bith brain and the preternatural power of his Bith senses to detect and resolve sub-microscopic structure, he eventually succeeded in creating a retrovirus that would transform normal midi-chlorians into long-lived maxi-chlorians.

But that was only the beginning.

With the patient, painstaking attention to the slightest, most insignificant detail that was his hallmark, Tenebrous had encoded his custom retrovirus with his most potent weapon: his own consciousness.

Once completed, Tenebrous had released the virus into his own bloodstream. It had spread throughout his body, infecting midi-chlorians in every one of his cells with gratifying alacrity. Not all his midi-chlorians, though, as the infected maxi-chlorians no longer fully functioned; to infect them all would have cut off his own connection to the Force. A partial severance of this connection was a necessary sacrifice, however, and through an extended process of trial and error, he was able to fine-tune the effect and confine it to the one sector of his Force powers he no longer needed-his ability to sense the motion of the future.

Of what possible use was the ability to see a future he already knew?

Source: The Tenebrous Way

Tenebrous's maxi-chlorians, as they enter Plagueis' body, block out Plagueis's own foresight. And as such, as Tenebrous's consciousness transfers into Plagueis's body, he is unable to gaze forward in time, and so he will never know how and why Plagueis will die:

The searing truth was driven home by the gathering darkness that clouded his borrowed foresight. Soon all he could see of the future was a hazy smear of shadow.... as the retrovirus he had become infected Plagueis' every cell. The retrovirus he had allowed to sacrifice his ability to gaze forward in time.... and had thus robbed his apprentice of his power to sense the future.

Which would seal his own doom as well.

Source: The Tenebrous Way

Desperate to escape Plagueis's death, Tenebrous yanks the maxi-chlorians out of Plagueis's body. He then hopes for his maxi-chlorians to simply fade out... only to realize that he didn't know how long it would take, and in the end, he realizes that his consciousness is reversed through time ad infinitum - he must relive his own death endlessly as his foresight is not eliminated, but twisted in upon itself. As he realizes that his consciousness is placed in an infinite time loop, he wishes he still has a mouth with which to scream:

Now wholly giving himself over to panic, Tenebrous turned his will upon undoing the damage he had done. With all his multiplied power, he yanked his maxi-chlorians back out from Plagueis' body in a spray of Force energy from his eyes, his mouth, the wound and every other cell. He had to think-he had to find a way out-or perhaps he didn't. Perhaps there

Perhaps the best he could hope for was the slow, inevitable extinction of his consciousness as his maxi-chlorians too faded and winked out. Then, at least, he would no longer have to squirm in the agony of his self-inflicted defeat....

If his maxi-chlorians were going to fade.

Because it dawned on him that he wasn't sure exactly how long the process should take, but he certainly didn't seem to be losing consciousness. He reached out with the Force-perhaps he could sense something. Anything. Or even contact Plagueis, somehow make his presence known, as his apprentice would never allow him to survive, no matter how reduced his powers might be....

But Plagueis wasn't here. Not only had Plagueis somehow vanished, Tenebrous could sense no trace of him ever having been here at all.... what was happening? How could this be?

The only trace of organic life Tenebrous could sense were some ancient mummified remains....

Of a Bith.

How long had he been here? How long would it take for every trace of Plagueis to vanish? Those remains were years old-decades, perhaps centuries old.

Tenebrous wondered, with dawning horror, if his retrovirus might have somehow mutated, if its effects on the maxi-chlorians might go somehow deeper than excision of foresight?

What if his eternal life would be.... this?

Or worse: what if his foresight hadn't been eliminated, but had been somehow twisted in upon itself? What if his remains were ancient because this was the thousandth time he had relived his death and the shattering revelation of his life-long self-deception.... what if this was the millionth time he'd relived it?

The billionth?

Then he knew, and at that moment he wished he still had a mouth, because he really, really needed to scream.

Dying, Tenebrous observed with mild surprise, was turning out to be not only pleasant, but wholly wonderful; had he ever suspected how much he'd enjoy the process, he wouldn't have wasted all these decades waiting for his foolish apprentice Plagueis to do him in....

Source: The Tenebrous Way