Darth Krayt formed the One Sith on Korriban before 30 ABY (after the Battle of Yavin). Krayt would keep the Order a secret all through the Second Galactic Civil War and the reemergence of the Lost Tribe of the Sith and Abeloth, silently observing the events that were happening in the universe at the time. Over time, Krayt nurtured the fledgling order, making it grow from some 50 members to a strong fighting force.These Sith were raised and trained on Korriban, used yorik coral for their lightsaber handles, were given black and red Sith tattoos from head to toe, and were extremely loyal to Krayt.
A century later, Krayt would order his agents to sabotage the Jedi and Yuuzhan Vong project, Project Ossus, which was a terraforming project that would utilize Vong techniques to fix up worlds that had seen the worst damage during the Yuuzhan Vong invasion years earlier. As Krayt's agents sabotaged the project, the planets which were supposed to be fixed up only became inhospitable, having their plant life snuffed out and inhabitants of the worlds receiving strange growths on their bodies.
Because of this, tensions rose between the Empire and the Galactic Alliance, resulting in another war. Emperor Roan Fel, however, wasn't fully committed to the war effort, and thus held back his Imperial Knights, intending not to get them involved. It was at this point that Darth Krayt chose to reveal his Sith Order, lending aid to the Empire and allowing them to defeat the Galactic Alliance in a matter of years.
Rise to Power
Now that the Galactic Alliance had been taken care of, Krayt intended to get rid of his only other enemy - the Jedi Order. Knowing the Jedi had fled to their ancient homeworld Ossus, Krayt sent one of his top enforcers, Darth Nihl, along with a striketeam of Sith, to the planet with orders to raze the Jedi's Temple and end their Order. The Sith and Imperial stormtroopers managed to kill many Jedi that day, and what few remained went into hiding, no longer a threat to Krayt and his One Sith.
As the attack was commencing on Ossus, Krayt took the rest of the higher ups in his Order and marched on the throne on Corsuscant, slaying the Imperial Knights guarding the "Emperor", as well as the "Emperor" himself. However, Krayt knew this was only a body double and not the real Emperor Fel, and made it his top priority to find the real one, tasking Darth Maladi with the important mission. As he made the Moffs swear allegiance to him, Krayt took his place on the throne, now the new Emperor of a Sith Empire.
Search For a Healer
Over the next seven years, Krayt consolidated his power and sent raiding parties out to hunt down the surviving Jedi, once again led by Darth Nihl. Eventually, Krayt would learn that a Skywalker had survived the attack on the temple seven years prior, and that he had an amazing healing ability, one that brought his master back to life from the verge of death. Krayt focused a lot of the Sith's resources and efforts into capturing this Skywalker - Cade Skywalker - as he had contracted an unknown disease that was slowly killing him due to the experiments he had been put under as a Yuuzhan Vong prisoner. Not wanting to give up his seat of power so soon after obtaining it, Krayt set a trap for Skywalker, eventually capturing him and attempting to convince him to join the Sith. At this time, many of his followers began to question Krayt, most notably his second-in-command, Darth Wyyrlok.
After a few weeks, Skywalker would escape Coruscant and eventually fall into the hands of Celeste Morne, an ancient Jedi who harbored a deep hate for Sith. Taunting Krayt and goading him into a trap, Morne, along with Skywalker and a force of Jedi and Imperial Knights, attempted to kill Krayt, stabbing him through the chest, hitting him with Force Lightning, and knocking him off a cliff. Believing their Emperor dead, the Sith fled the planet, all except Wyyrlok. He found Krayt at the bottom of the cliff, barely clinging to life. However, instead of bringing him back for medical help, Wyyrlok shocked him with his own Force Lightning, believing he himself had killed Krayt and become the new ruler of the One Sith.
Darth Wyyrlok knew that the majority of the One Sith wouldn't willingly follow him, so he had Krayt's body transferred to a stasis chamber on Korriban in case any Sith had wondered where their Emperor had gone. Since Wyyrlok had been at Krayt's side through his many trips in Healing Trances throughout the years, no one wold think twice about the idea of Krayt going into stasis to "heal himself" while Wyyrlok handed out his orders for him.
Despite this, many Sith began to suspect that Wyyrlok had indeed killed Krayt and that he just wanted the throne for himself. To gain his trust, Wyyrlok appointed Darth Nihl as his Hand and tasked him to Korriban. Afterwards, Wyyrlok ordered the complete annihilation of the Mon Calamari species through the poisoning of their oceans, believing doing so would keep the galaxy in check.
After learning of a secret meeting between Imperial and Jedi forces, members of the One Sith would interrupt Roan Fel and the Jedi Order on Agamar, attacking both sides in an attempt to get rid of both parties at once. Although Fel escaped the battle, the Sith were able to capture his daughter, Marasiah Fel.
On Korriban, Darth Nihl visited Krayt's tomb only to find his armor without any body inside. Believing Darth Talon had killed him, he engaged the female Sith in a duel. However, Nihl would find out the truth about Darth Krayt soon enough; he had survived and had been reborn stronger than ever. Calling out to all the dark side users in the galaxy, Krayt made his rebirth known to the One Sith, prompting an inevitable confrontation between himself and Wyyrlok with the results of their duel being rule over the Sith and their Empire.
Darth Krayt made his way back to Coruscant with Nihl and Talon and challenged Wyyrlok for rule of the Empire. Many of the Sith present were loyal to Wyyrlok, some to Krayt, and others would pledge themselves to whoever won, the stronger Sith. The two engaged in an impressive duel, both wielding powerful Force attacks and utilizing amazing lightsaber skills. The fight would see Darth Krayt the victory, however, and he would assume his role once again as ruler of the One Sith.
Going into Hiding
As the war with the Fel Empire, Jedi Order, and remnants of the Galactic Alliance came to an end, the One Sith engaged their enemies in a massive battle on Coruscant, which would decide the fate of the galaxy at large. After brutal fighting, Darth Krayt fell in battle to Cade Skywalker and the Sith Empire was defeated. With Krayt and Wyyrlok gone, Darth Nihl was next in line as leader of the Sith.
Nihl ordered the remaining Sith to disperse, go into hiding, and infiltrate any governments they could in order to have power should they rise again. The One Sith complied, going into hiding and dispersing among governments around the galaxy.
Some years later, Darth Wredd, a Sith apprentice turned Dark Lord, began to identify and kill members of the One Sith that had gone into hiding as part of his plan to rid the Sith from the galaxy for the pain they caused him. After hunting him down to the planet Mala, the remaining One Sith forces battled against the Galactic Triumvirate who were also trying to catch Wredd. After a long and brutal battle, the One Sith had been defeated, ending their order once and for all.
As leader of the One Sith, Darth Krayt implemented ranks and divisions for his Sith Order because he had many Sith to work with after quitting the Rule of Two. The ranks are as follows:
The second in command of the One Sith. Members of this position provided services to the reigning Dark Lord of the Sith such as caretaking, protection, and advising. They would be the one to take care of Darth Krayt whenever he entered stasis, relaying his orders to the rest of the Sith and helping Krayt to heal himself. This position was held by Darth Wyyrlok.
The Sith Lord who held the title of Fist in the One Sith was the supreme commander of the Sith's military forces. Darth Nihl and Darth Stryfe held this title.
Much like how it was under Emperor Palpatine's reign, the title of Hand was given to the deadliest assassins in the Empire. They had to be completely loyal to Darth Krayt and acted as his personal assassins as well. Darth Talon and Darth Nihl held this title.
As the title suggests, this position was given to the Sith Lord who was in charge of all the Sith tomes, artifacts, scrolls, and records of the Order. Darth Wyyrlok held this position as well.
Unlike their predecessors with the same rank, Darth Krayt's Inquisitors were Sith Lords who specialized in interrogation and torture. The only named Inquisitor in the One Sith is Darth Havok.
Intelligence and Assassination Unit
Members of this unit specialized in gathering intelligence on whomever Darth Krayt wished and assassinating certain political targets. This unit was lead by Darth Maladi.
After being reborn, Darth Krayt introduced his Sith Troopers into the ranks of the One Sith. Sith Troopers were identified by Darth Krayt at birth for their strength in the Force and shipped off to Korriban to be cybernetically augmented and enhanced. Krayt linked himself telepathically to all the the Sith Troopers, ensuring their complete loyalty to him and only him.
They are powerful Force wielders and skilled lightsaber duelists, and have also linked themselves to advanced starships through the power of the dark side to become deadly skilled and dangerous pilots.