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)

The disappearance
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)

Historically 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, and incorporated it to reconcile his reasons for Vandekirian's betrayal. The Doctor hypothesised that Vandekirian had either sabotaged the mission on behalf of Rassilon, or he had just succumbed to madness. (AUDIO: Omega)

According to another account, Fenris the Hellbringer, (COMIC: Star Death) an agent for the Order of the Black Sun, (COMIC: 4-D War) sabotaged Omega's Starbreaker. This was to ensure 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's interventions dispatched Fenris and the time experiments succeeded. Rassilon prevented disaster from overtaking the other three Starbreakers (COMIC: Star Death)

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)

Aftermath
Rassilon wept over Omega's death. (PROSE: Lungbarrow)

Omega's Star was renamed "Rassilon's Star". (AUDIO: Zagreus)