I am creating this for my own use, as much as I am using it to spread the lesser known feats and accolades in Valkorion's name.

As a child of ten years, Vitiate learns how to manipulate Holocrons in an advanced way:

All holocrons are based on a complex yet elegant technology: a lattice of organic crystals woven together at a microscopic level. The crystal lattice can store vast amounts of information, as well as replicate the appearance and personality of the holocron's creator as a gatekeeper who will guide students in their training.

In his youth, Valkorion--then known as Tenebrae--discovered a way to twist and pervert the lattice so it could capture the spirit of powerful Force users, locking them in a metaphysical cage. He used the weapon on Dramath, his tyrannical father, and trapped him inside the holocron for centuries.

Credit: Dramath's Holocron Codex Entry

Surpassing Darth Bane, where he struggled to painstakingly achieve the creation of a regular Sith Holocron, despite having the blueprint of Freedon Nadd's own Holocron:

Using Freedon Nadd's Holocron as a blueprint, he had re-created the intricate matrix of lattices and vertices that were the key to storing nearly infinite amounts of knowledge in a data system small enough to fit in the palm of a hand. It had taken months to gather and fashion the rare crystal into the filaments and fibers of the interlaced network, followed by weeks of delicate and painstaking adjustments. The matrix had to fall within highly exacting specifications, and Bane had spent hundreds of hours making thousands of precise, subatomic alterations through the power of the Force to ensure that each crystalline strand was properly in place.

Once the crystal matrix inside the Holocron was ready, he had carefully transcribed the ancient symbols of Sith power onto the pyramid's surface. The markings were part of a powerful ritual that was critical to maintaining the stability of the matrix after it was infused with the energies of the dark side. Unfamiliar with the exact purpose or meaning of the arcane glyphs, Darth Bane had once again used Nadd's Holocron as his guide, studying the markings etched on the surface, then copying them exactly on his own creation.

But when he tried to activate the Holocron by channeling his power through it, the matrix imploded, collapsing in on itself and reducing the artifact to a pile of glimmering dust in a crackling white flash.

He had tried again several months later, only to be met with the exact same result. Forced to admit that the secret of crafting Holo-crons was still beyond him, Bane had begun a campaign to discover everything he could about the powerful talismans. With Zannah's aid, he accumulated a vast wealth of knowledge on the subject.

Credit: Darth Bane: Rule of Two

This is prior to a century of growth and mastery:

"It took Tenebrae three more years to gain control of the rest of Nathema. Dramath's firstborn son fled rather than face his formidable half brother, but other powerful Sith sought to seize the empty throne. All fell before the dark prodigy, and with each victory he grew more powerful and more ruthless."

Credit: Darth Nyriss, SWTOR Revan

Vitiate enacts the Ritual of Nathema, becoming multiple times more powerful:

The plan was both horrifying and brilliant. In addition to becoming more powerful than Meetra could imagine, Vitiate could blame the extinction of his homeworld on the Jedi, further panicking the remaining Sith worlds.

Credit: SWTOR: Revan

This places Vitiate above Darth Nihilus, as of the completion of the Nathema Ritual. Darth Nihilus being an unquestionably more powerful being than even a powerhouse such as Galen Marek, by comparison of telekinetic feats:

Nihilus pulled the Ravager, and other ships in his fleet, out of the gravity well of Malachor V:

"This ship, is it his weakness? It should not exist, yet it cruises the darkness between the stars. He tore it from the mass shadows of Malachor, along with his fleet, that is a measure of his power."

Credit: Colonel Tobin, KOTOR 2

NOTE: The circumstances surrounding this are insane given the following:

The storms of Malachor V are enough to draw down even the strongest of ships. Once on the surface, there is little chance of escape.

Credit: KOTOR 2

In comparison, it took Galen Marek passing out to pull an already falling ISD down to the surface of Raxus Prime:

With his legs braced firmly in the trash, he reached as deep as he had ever reached into the Force, and then went farther still, feeling as though a mighty chasm had opened up under him and his mind and will plunged down into it. The chasm filled. His mind opened. The physical existence of the Star Destroyer slid painlessly inside.

....

The apprentice ignored it all. While the illusion held, he moved his hand a very slight distance to his right. The sensation of containing a vast, million-ton machine in the tip of one finger was deeply disorienting. He felt as though every muscle fiber, nerve, and bone groaned along with the metal seams and joints of the ship. What it felt, he felt, too, and even a small acceleration had a profound effect on such a large scale. It resisted with all the momentum it possessed. Hatches swang open; rivets popped; bulkheads twisted; pipes burst.

The Star Destroyer didn’t appear to have moved much in the sky. It was still coming in low on the horizon, aiming to pass over him and strafe him from above. He shifted his hand a second time, but instead of changing its course he mistakenly gave it a slight tumble. He needed to apply the Force the right way for this to work, taking the growing forces of friction and the shifting of its center of gravity into account. A spinning Star Destroyer would do more damage than one burying itself nose-first into the cannon and its superstructure. Damage was good, when it came to destroying the Emperor’s handiwork, but too much damage could destroy him and perhaps the Rogue Shadow as well under a deadly rain of molten shrapnel.

Bring it down in one piece, he told himself. Bring it down hard.

The ship growled and squealed in metal torment. He was getting the hang of it; he could see how its course was slowly shifting. As wide across as his outstretched hand now, it was hitting the atmosphere at a steeper angle than he had intended, burning bright red and already gouting a trail of black smoke and sparkling debris. He became aware of a sound communicated through his feet: a rumbling much deeper and more sustained than the pounding of the cannon, which had fallen silent after the firing of the third projectile. The Star Destroyer’s incomplete frame was acting like a giant tube, and the atmosphere was resonating inside. His whole body sang with it.

More. The Star Destroyer was really picking up speed now. The thickening atmosphere had a slight braking effect, but nothing could prevent the inevitable. It was going to hit soon. A wild exodus of droids ran past him, fleeing the crash site. The TIE fighters it had launched raced ahead of the chaotic atmospheric waves it generated. He ignored them and concentrated on shifting ground zero as close to the cannon as he could.

Sparks danced in front of his eyes. The edges of his vision faded to black. Light and dark swirls spun around him, wraith-like. He felt momentarily faint and wondered if it was possible to dissolve into the Force. He was a speck caught in the updraft over a forest fire—yet somehow he had the audacity to try to command the fire to do his will.

Who did he think he was?

A sudden panic almost made him lose control. The Star Destroyer, now a burning, shrieking meteor, filled his entire forward vision. The hull was peeling away in fiery, golden strips, each one weighing hundreds of tons, exposing the darker skeleton beneath. It looked like a death’s-head, a ghastly mask not dissimilar to his Master’s, but one molten like lava.

....

The apprentice clutched at it with desperation, needing to regain control of the Star Destroyer lest it tear itself apart and disperse the impact. He needed to find his focus again, to ignore the feeling of dissolution eating at the edges of his self, and to tip the balance of power back toward him.

....

He gritted his teeth and snarled at the sky. The dead weight of the Star Destroyer shifted one last time, changing its angle of descent just enough to hang together those last few hundred meters, but not enough to risk bouncing. Only seconds remained before it hit and it was still getting bigger. It was impossible that the sky could contain so much metal!

Abandoning his control over the ship, knowing there was nothing now that he could do to alter its course, the apprentice staggered backward, dazed. The Force fled from him, leaving him wrung out and drained. With a sound like the world ending, the Star Destroyer completed its first and final journey. It hit the cannon, exactly as it was supposed to, and the sky turned white. The ground buckled beneath the apprentice’s feet. He pinwheeled, unable to find his balance, as a tsunami of junk and waste rose up ahead of him and blotted out the sun.

Credit: The Force Unleashed

Galen goes on to engage Emperor Palpatine in a battle of desperation for both sides:

"You fool!" snarled the Emperor, sending another wave of Sith lightning into Kota's back. "He will never be yours."

Kota fell with his arms upraised, and the apprentice knew that it wasn't over yet. The moment of truth had arrived.

Without hesitation, he stepped between Kota and the Emperor, taking the full brunt of the Sith lightning into his own body.

The pain was incredible, searing every nerve back to its individual cells, skewering each of them on white-hot needles. He had never before felt anything like this. He wanted to recoil from the source, to curl into a ball and let unconsciousness take the pain away, but somehow he stayed standing, seeing the world through a crackling blue light, and even took a step toward the Emperor.

"Go!" he hissed at Kota. "Hurry!"

The general hesitated only for a moment. He, too, had seen a glimpse of the future, the apprentice remembered. He knew that it came down to a simple choice: him and the Rebels or the apprentice and darkness forever. Gathering up the Rebels, Kola ushered them toward the descending ship.

Another staggering, painful step and the Emperor was within the apprentice's reach. With shaking fingers, he took the old man's bony shoulders in his hands and gripped them tight. The Sith lightning spread to engulf the two of them, fueled by both their desperations. The Emperor tipped back his head and howled in lascivious pain. Darkness threatened to envelop the apprentice's mind, but he clutched to consciousness with feverish will. He had to see this through. He had to.

Credit: The Force Unleashed

Darth Nihilus, however, had achieved his telekinetic feat prior to absorbing numerous planets of their Force energy:

The Ravager was hauled from the gravity well at Malachor V by its new master. He used it to escape imprisonment on Malachor V.

Credit: KOTOR 2

Meaning, Nihilus' feat was massively pre-prime. His growth due to planet draining being immense here:

While illness cannibalizes his body, a vast emptiness devours his humanity. Then, when all seems lost, he discovers that his emptiness hungered. The first time, he fed it unconsciously. Draining another being's life is frightening, nauseating, but for a euphoric moment, the memories, the illness, the hunger disappeared.

But it proves insatiable. The more he indulges it, the shorter the satisfaction lasts and the more severe the hunger becomes. He begins feeding relentlessly, still always craving. Existence again becomes unbearable, but then hope materializes. A blind Sith Lord appears. She has felt his existence like a gaping wound in the Force. The Force, she explains, fuels his hunger, and she will show him how to devour worlds. The Sith and the Force are meaningless to him, but the hunger must be appeased.

Darth Traya indeed teaches the newly christened Darth Nihilus to harness his life-craving gift to radical heights – so effectively in fact, that Nihilus saps Traya's powers in a calculated double cross. However, as Nihilus greedily consumes entire planets' life energy, the dark side macerates him even faster. But no matter. Nothing matters except his hunger. Before it devours him totally, Nihilus uses its power to displace his persona into his robes and armor. As his useless body disintegrates, he becomes living primitive intention; at last, the whole of the galaxy becomes food – for Nihilus has become the hunger.

Credit: KOTOR Campaign Guide

Nathema, was not merely extinguished, it was rendered a Void in the Force:

Lord Vitiate orchestrated the sorcery and the planet Mediraas was consumed by the largest dark side nexus the galaxy would ever see. When the ritual ended, Lord Vitiate emerged as the only survivor. The pain, energy, and suffering of every living entity on the planet fueled his power and would prolong his life for centuries. The lifeless planet of Medriaas became a Void in the Force and was erased from history. From that moment forward, the world would forever be known as Nathema, birthplace of the one and only Sith Emperor.

Credit: SWTOR: Encyclopedia

This Void in the Force was felt within the Emperor himself:

As he spoke, the dark circles of his eyes seemed to fill with a swirling red mist, and for a brief instant the Emperor gave Scourge a glimpse of his true self.

Scourge cried out in anguish as the Emperor’s mind brushed against his, then he collapsed to the floor, shaking like a child. The touch lasted less than a second, but in that time he witnessed indescribable horrors that dwarfed anything the dark side could conjure even in his worst nightmares. And beneath the formless terrors lurked the unbearable Void, the pure emptiness of total annihilation.

Credit: SWTOR: Revan

The Void in question, was immediately closed upon the Emperor's true death. Indicating that Vitiate's mere existence, even as a spirit, caused the Void in the Force:

The ecological revitalization of Nathema has been nothing short of miraculous. Vibrant plant life has not only taken root but flourished, dramatically altering the planet's once-desolate landscape.

Force-users no longer experience the unsettling "wrongness" that they once felt whenever setting foot on the planet. The air has become fresher, and the weather more dynamic. That this has all occurred since the death of Valkorion cannot be a coincidence, though experts cannot agree on any precise environmental mechanism that could have brought about such an abrupt change.

Credit: Nathema Reborn Codex Entry

Something reiterated here:

Ironically, all of the Zealots still alive owe their survival to their master's death and the subsequent blooming of life on the planet's once-barren surface; otherwise, they would all have starved long ago.

Credit: Zealots Without A Master Codex Entry

So as of the completion of the Nathema Ritual, we have Tenebrae becoming a Void in the Force that hungers for more and more power. Power which he gains, constantly, for a millennia after Nathema:

In return, the Emperor draws on his servant's strength in the Force and body to feed his ever-increasing power, leaving the servants withered and frail.

Credit: SWTOR Encyclopedia

As of Knights of the Old Republic, Vitiate is the primary source of an imbalance in the Force that isn't seen again until the days of Emperor Krayt:

In the Legacy era, there is no balance to the Force. Instead, the dark side of the Force holds sway, and the influence of evil can be felt everywhere. With Sith Lords in every sector and at the heads of Imperial space fleets, the influence of the the dark is stronger than ever, and far more visible. The Sith act overtly, making it clear that they rule the Empire and, by extension, the galaxy. Not since the days of Darth Revan and Darth Malak have the Sith ruled so convincely. The dark side of the Force clouds everything, and the galaxy's future is harder to see than ever before.

Credit: Legacy Campaign Guide

As an aside to this, it's stated that the dark side shrouds the galaxy so intensely that Revan and Bastila Shan couldn't sense the disturbance in the Force caused by the destruction of the Jedi Enclave on Dantooine:

"We should have felt a disturbance in the Force when the attack came. The fact that we did not is a bad sign. I fear the dark side is growing stronger, casting shadows our vision cannot pierce."

Credit: Bastila Shan, Knights of the Old Republic

Infact, the psychic misery of the Galactic Republic was so strong at this time, that the Sith could actually feed upon it:

The Sith seemed to have achieved the victory they had long sought. Less than one hundred Jedi, nearly all of them in hiding, survived the Sith's purge. Many citizens took this unimaginable event as proof that the Force had turned malevolent, or that cosmic balance was nothing more than a story for children. Despair was palpable, and many planets offered only halfhearted and ragged defense against Sith attackers. Planets throughout the slice surrendered, and the Sith fed on the psychic misery of a shattered Republic.

Credit: The Essential Atlas

The Emperor then captures Revan, feeding on his immense powers to fuel his own:

REVAN COULD FEEL THE EMPEROR FEEDING ON HIM, drawing on his power to sate his endless hunger.

Credit: SWTOR Revan

As well as constructing the Dark Temple, where he imprisoned powerful Sith; living and dead, which he could drain from constantly:

The temple became a nexus of dark side energy as the Emperor performed rituals within, drawing strength and knowledge from his captives.

Credit: SWTOR Encyclopedia

Vitiate, as of SWTOR, is stated to be such a supreme powerhouse, that he made the Dread Masters insignificant by comparison:

"The Dread Masters affected Oricon. Even their combined power was insignificant compared to the Emperor."

Credit: The Empire's Wrath, Shadow of Revan

As a matter of fact, it is outright stated by one of their own that Vitiate's power alone was enough to 'control' them, evidently speaking to them telepathically:

"The Emperor was. . . powerful. He alone was worthy. But now he is silent. None can control us."

Credit: The Dread Masters, SWTOR

The Dread Masters are immensely powerful, having harnessed and perfected the Phobis Devices:

Fear is a powerful weapon, and those who wield it can rule the galaxy. The Phobis devices were constructed for just that purpose. Brimming with dark energy, their mere presence was enough to spark crippling horror in the bravest of minds. Many Sith who meditated on the devices were driven mad by their attempt. Only the lost Sith Dread Masters successfully harnessed and perfected the Phobis devices' frightening power.

Credit: Phobis Devices Codex Entry

This power gave the Dread Masters the ability to terraform planets, much in the same way they'd done to Oricon:

"Our ships will deliver our devices across the galaxy; Dromund Kaas, Nar Shaddaa, Coruscant. Populations will be infected and planets overgrown."

Credit: Calphayus, Dread War

As well as driving everyone incurably insane:

"Our research suggests that very few members of this force are willing volunteers; most are former Republic and Imperial personnel whose minds have been irreparably dominated by the power of the Dread Masters. Efforts to question or treat captured members of the Dread Host have been entirely unsuccessful. The constant psychological manipulations of the Dread Masters have rendered their followers both completely loyal and incurably, paranoically insane."

Credit: Master Gnost-Dural

The depth of insanity inflicted is so severe that even non-Force sensitives would take at least ten blaster shots to the chest without feeling any pain:

"He’d passed all the psych tests, resisted every mind-altering device and chemical they could find. Plenty of qualifications... I never got to ask them how things went from bad to nightmarish; he’d spread pieces of them all over the place by the time I got there. The guards were dead, too, but just the normal, murdered kind of dead. I guess they didn’t merit the special treatment, even though they must’ve shot the kid five times apiece. He was just standing there, full of holes, smiling that same shiny kid smile he had when they first brought him in."

Credit: Jonas Balkar, The Search For Oricon

The populations of the planets listed above?

Dromund Kaas:

Over the following centuries, its population grew into the millions–proud Imperials working toward the glory of the Sith and slaves submitting to Imperial will.

Credit: Kaas City Codex Entry

Nar Shaddaa:

85,000,000,000

Credit: The Essential Atlas

Coruscant:

1,000,000,000,000

Credit: Coruscant, Center of the Galaxy

After the Dread War, Tenebrae returns on Yavin IV due to Revan's machinations and attacks the planet of Ziost. It has been recently established that Tenebrae's death wave was not the result of a ritual. Instead, the only ritual Tenebrae ever used in the Sith throneworld's decimation was designed to absorb the death that had occurred as a result. Meaning a death wave that destroyed Ziost's surface, was something Tenebrae as an incorporeal spirit could do of his own power, after having recovered enough from his slumber on Yavin IV.

NOTE: This makes complete sense, as we've already established that Tenebrae in a massively weaker state than his prime by this point, was vastly beyond the scope of Darth Nihilus' powers.

Now fully assuming the mask of Valkorion, Immortal Emperor of Zakuul. Tenebrae is a vastly more powerful Entity than he'd ever been before:

With each rebirth, he grew stronger.

Credit: Valkorion Codex Entry

Something Tenebrae himself reiterates:

"Ziost. The world where everything changed. For me, you... the galaxy."

They (Ziost's inhabitants) died opening my eyes to the truth. I have passed beyond death's reach. Voices... Hands... Children... I no longer require those crude vessels. At long last I am truly free."

Credit: Tenebrae, KOTFE

Meaning that every single thing you read prior to this, is vastly pre-prime.

At this point, Valkorion has achieved the power to face and handily defeat the Outlander, Vaylin and Arcann. Dominating all three, one-by-one, with apparent ease, despite them weilding an incredibly dangerous weapon:

But the same corrupted technology Valkorion used to vanquish his father could also be turned against him, permanently imprisoning his immortal spirit... or obliterating it from existence.

Credit: Dramath's Holocron

Yet, upon the destruction of his spirit, he causes a shockwave in the Force that was felt by Force-users across the galaxy. So strong it threatened to black out a powerhouse such as Satele Shan:

Although each and every one of the Zealots felt Valkorion's death vividly through the Force, all of them refuse to believe that it has actually occurred.

Credit: Zealots Without A Master

Satele personally notes this:

"I felt the destruction of his spirit from half a galaxy away--a shockwave in the Force that ripped through me with such power that I nearly blacked out. I've never felt anything like it before, but I knew instantly what it meant.

At first, I was afraid to believe it was true. But Darth Marr's spirit felt it, too."

Credit: A New Age, KOTET

It's not too late to drop Sheev's fresh corpse and take my hand...