Osiris

Osiris was the last great leader of the Osirans of Phaester Osiris, venerated in Egypt, on Earth, as one of their most important gods. He was murdered by his destructive fellow Osiran Sutekh, and his murder avenged by his son Horus — both facts recorded in the Egyptian myths.

Rule
Osiris ruled Phaester Osiris alongside Sutekh; though Sutekh was his brother, (PROSE: The Sands of Time) the Legend of Sutekh claimed that Osiris had been Sutekh's father, instead. (PROSE: Background) Sutekh and Osiris had been specifically bred to channel the energy of Ra. (AUDIO: The Ship of a Billion Years) Isis was Osiris' sister and wife. (PROSE: The Sands of Time)

Murder by Sutekh
Sutekh, having decided to kill his brother, lured Osiris into a space capsule. Sutekh and his sister/wife Nephthys were able to propel the capsule into space and stop Osiris from mentally projecting himself. He suffocated in the vacuum of space and was unable to project his mind over the great distances. While Isis was searching for him, he was eventually able to project part of his mind into the pilot of her ship, giving birth to his psi-child Horus. (PROSE: The Sands of Time)

The Legend of Sutekh claimed that Sutekh's punishment, once Osiri's son Horus caught up with him, was to forever bear the body of Osiris on his shoulders; (PROSE: Background) the Fourth Doctor would later discovere that Horus had trapped Sutekh in eternal paralysis beneath a pyramid, a type of tomb. (TV: Pyramids of Mars)

Return
However, Osiris returned to Earth years later in the hopes of recapturing it. (COMIC: Spiral Staircase)

Legacy
While battling Vlad the Impaler's soldiers in Wallachia on 17 June 1462, Erimem swore by Osiris. (AUDIO: Son of the Dragon)

While confronting the vampire Joanna Harris, the Eighth Doctor implied that Osiris had been one of his past identities; James Court (who witnessed said confrontation) thought this to have been a joke but couldn't be sure. (PROSE: Vampire Science)