Fenris

"Fenrius the Hellbringer" was a time traveller who, by several accounts, was responsible for the disappearance of Omega during the experiments which created the Eye of Harmony, pursuing the agenda of the Order of the Black Sun.

Biography
Fenris travelled back in time to sabotage the initial Gallifreyan time travel experiments at Qqaba. He caused the stasis haloes on Omega's ship to fail, making Omega fall into the black hole Qqaba. Rassilon's intervention, however, dispatched Fenris and the experiment worked. Fenris's discarded belt provided the much-needed technology to use the new energy and could also be used as a form of directional control. (COMIC: Star Death)

Twenty years after the loss of Rassilon, the Time Lords tried to free Fenris from the Zone of No Return and used the parahuman Viridian to find out he was working for the Order of the Black Sun. But before they could learn anything else, they were attacked by members of the Order of the Black Sun, who killed Fenris and some other Time Lords before vanishing. (COMIC: 4-D War)

According to one source, however, 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 as vengeance for causing Omega's apparent death. (PROSE: Gallifrey: A Rough Guide)

Other accounts of the Disappearance of Omega did not feature Fenris. (AUDIO: Omega, et. al)