Omega's sacrifice

The disappearance of Omega was an event at the beginning of Time Lord history in which Omega was lost in a supernova while securing the power source that the Time Lords required for time travel. Omega was presumed dead, but he survived many of thousands of years in the anti-matter universe. (TV: The Three Doctors)

By different accounts, the star that Omega was lost in was Qqaba (COMIC: Star Death, PROSE: The Infinity Doctors), Polyphilos, (PROSE: The Scrolls of Rassilon) or the Star of Jartus. (AUDIO: Omega) It was located 9.6 light-years from Gallifrey in the constellation of Ao, in the Veil Nebula, (PROSE: Lungbarrow) in an area known as the Sector of Forgotten Souls. (AUDIO: Omega) It was a Q star which contained tremendous potential energy because of its axial spin, (PROSE: The Scrolls of Rassilon) and the home of the Solarians. (PROSE: Prisoners of the Sun)

Overview
The stasis halo of Omega's Starbreaker, the Eurydice, was sabotaged, exposing it, and the Starbreaker's crew, to the fury of the black hole. There were several accounts concerning the details and reasons for the sabotage, (COMIC: Star Death, AUDIO: Omega) though one Gallifreyan wrote that Omega had "got his sums wrong" and was simply not within a safe distance when the star exploded. (PROSE: A Brief History of Time Lords)

One image included within book about the history of the universe depicted Omega standing triumphant as he slowly crumbled, with a black circle in the background. Just like the Gallifreyan before it, the circle crumbled. (PROSE: The Whoniverse)

The fault of the deceitful assistant
Historical accounts depicted that Omega's assistant, Vandekirian, had sabotaged the mission on behalf of Rassilon. However, feeling guilty over his betrayal, he cut off his own hand as recompense. Omega did not accept it, and cut off his other hand, placing it in his stellar manipulator, which was later to be known as the Hand of Omega. Upon which, the Eurydice was destroyed due to Vandekirian's actions. (AUDIO: Omega) According to the Seventh Doctor however, the Hand of Omega was "not his hand literally", but had instead only received this nickname figuratively, "because Time Lords have an infinite capacity for pretension". (TV: Remembrance of the Daleks)

It was rumoured by the time of the Doctor that Omega's "death" had been arranged by Rassilon, who had tasked "Omega's assistant" with betraying him. (COMIC: The Lost Dimension) In his own distorted memories, Omega recalled that Vandekirian was at least partly responsible for the sabotage. He had discovered that detonating the star would jeopardise the existence of a race called the Scintillans. When he realised what would happen, his guilt caused him to destroy his own hand, since its activation required his palm print.

However, Omega cut off his other hand to activate it. Although, the Fifth Doctor revealed that the history with the Scintillans was a corruption of his own memories, over the guilt of accidentally killing the race himself, that Omega had access to after gaining some of the Doctor's biodata; the Doctor assumed Omega simply incorporated the story into his thoughts to reconcile his reasons for Vandekirian's betrayal or to believe his exile was because of something like genocide, instead of a simple mistake.

The Doctor hypothesised that Vandekirian had either sabotaged the mission on behalf of Rassilon, or he had just succumbed to madness. (AUDIO: Omega) The Eleventh Doctor appeared to believe Rassilon was to blame. (COMIC: The Lost Dimension) Another account claimed Rassilon had an even more direct role in events, claiming he had deactivated Omega's forcefields seconds before the star exploded. (PROSE: The Scrolls of Rassilon)

The fault of Fenris
The Fourth Doctor, however, believed Rassilon was innocent and subscribed to the account that blamed a man known as "Fenris the Hellbringer" with the "death" of Omega.

According to this telling, Fenris sabotaged Omega's Starbreaker. This nearly meant that the initial Gallifreyan time travel experiments never came to pass, so the Time Lords "[would] be annihilated before they [had] even come into existence". However, Rassilon intervened, dispatching Fenris; thus, although Omega was lost, the time experiments succeeded. According to this account, Rassilon prevented disaster from overtaking the other three Starbreakers (COMIC: Star Death) before having a chance to cry over Omega's death. (PROSE: Lungbarrow)

Most accounts suggested that Fenris had been an agent for the Order of the Black Sun and acting deliberately, (COMIC: 4-D War) but, according to one source, the man who caused Omega's death was a "temporal tourist" of no importance, who had gone back to Gallifrey's ancient history to witness the historic moment of Qqaba's detonation. He was only given the melodramatic name of "Fenris the Hellbringer" in later retellings of the event, postdating his almost immediate obliteration by Rassilon. (PROSE: Gallifrey: A Rough Guide)

Omegon the emperor
According to yet another account, Omegon actually survived the original experiment which granted the Time Lords time travel, and established himself as the Emperor of the new society. However, the other Time Lords plotted against him. They attempted to destroy him using the very power he had harnessed, and, when he proved invulnerable to it, they instead imprisoned him in a time trap in a sealed bubble of reality. (PROSE: K9 and the Time Trap)

One tour operator from the post-War universe was of the opinion that the "Omegon" peddling this distorted version of Omega's tale was a mad impostor rather than the real Omega. (PROSE: Gallifrey: A Rough Guide)

Aftermath
Rassilon publicly wept over Omega's death. (PROSE: Lungbarrow) Rassilon received most credit for everything Omega had done despite having had a relatively minor role in their discoveries. (PROSE: The Legacy of Gallifrey) Omega's Star was renamed "Rassilon's Star". (AUDIO: Zagreus)

Accounts that foregrounded the version of events involving Fenris created the disappearance of Omega as a starting point for the First Great Time War. (COMIC: 4-D War, et. al)