Horus was an Osiran related to Osiris and Sutekh. By one account, he was the hybrid formed when Osiris' biodata was merged with that of Eliza. (AUDIO: Body Politic) He was also identified as Harpocrates, the God of Silence. (PROSE: Head of State)
Birth
After Osiris was killed by Sutekh, Justine il-Isis searched for the pieces of Osiris' body and, with the help of Anubis and Thoth, attempted to reconstitute him. However, since one vital piece of his biodata was missing, Cousin Eliza of Faction Paradox offered herself as an alternative donor and became the entity known as Horus. (AUDIO: Body Politic)
By another account, Horus was the psi-child produced when Sutekh stranded Osiris in space and Osiris projected part of his mind into the pilot of Isis' search ship. (PROSE: The Sands of Time)
Rise and leadership
A tribunal was convened to decide whether Horus or Osiris should inherit the throne. It was judged by Geb, father of Sutekh and Osiris. The tribunal ended when Horus accepted Sutekh's challenge of ritual combat to decide the matter. (AUDIO: Ozymandias) The Court was led to believe 740 Osirans were led by Horus in battle against Sutekh, who was finally defeated in Egypt. (TV: Pyramids of Mars, PROSE: The Sands of Time) However, in a complex time travel-based plan formulated with Cousin Justine, Horus really led the Osirians to combat Lolita on the Homeworld. (AUDIO: The Judgment of Sutekh)
Heavily-wounded, Horus was put in suspended animation in the Black Pyramid on Beta Osiris. After this, most believed him to be dead, but he didn't actually die until the cryogenic systems failed in the 16th century. The Fourth Doctor and Sarah Jane Smith found his corpse in the 2560s. He resembled a cow. (PROSE: Scarab of Death)
Worship
He worshiped by many cultures. The Egyptians of Earth worshiped him amongst the pantheon of gods, many of whom were named after Osirans.
The native Martians also worshiped Horus. Forming the Holy Order of Oras, they followed the teachings of the Book of Oras. The Order's teachings were of a non-violent nature and asked its followers to abandon such means. The Ninth Book of Oras was the Rebirth of the Father. (PROSE: GodEngine)
When inducting his earlier self into the Faction, Richard Francis Burton made himself swear by "Horus, the reborn child from whose sacrifice our Faction owes its own rebirth". (PROSE: Head of State)