Horus was an Osirian related to Osiris and Sutekh. By one account, he was the hybrid formed when Osiris' biodata was merged with that of Cousin Eliza and modified by Lolita. (AUDIO: Body Politic) Horus was also called Harpocrates, god of Silence. (PROSE: Head of State)
A Sutekh-possessed Marcus Scarman claimed that Horus was Sutekh's "brother", just as Marcus was brother to Laurence. (TV: Pyramids of Mars) However, there were different accounts about what precisely this entailed. One simply treated Horus as biological brother of Sutekh; after Sutekh destroyed the Osirian homeworld Phaester Osiris, Horus swore to show himself better than his brother and sought justice, instead of revenge. (PROSE: The Enigma of Sisterhood)
However, other accounts agreed that Sutekh's actual brother was Osiris, whom Sutekh had betrayed, with Horus being a new identity adopted by the essence of Osiris as it returned in the body of a mortal. (PROSE: The Sands of Time, AUDIO: Body Politic) By one such 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)
According to another, after Osiris was killed by Sutekh, Justine il-Isis of Faction Paradox sought to nullify Sutekh's claim to the Osirian throne by arranging for his resurrection. She searched for pieces of every part of Osiris' body and biodata, which she then brought back to Anubis, the most skilled flesh-sculptor in the Osirian Court, so that he could recreate a new body for Osiris.
However, vital pieces of his biodata were missing. As Justine had unwittingly contaminated the rest of Osiris's biodata with Faction Paradox influences while she collected it, only another memebr of the Faction was a usable donour; Cousin Eliza of Faction Paradox offered herself. She was placed in a tank and "pumped full" of the recovered Osiris biodata. Lolita covertly added a few drops of her own blood to the tank, in the hope that she could use this connection to control Osiris later on. (AUDIO: Body Politic)
The resulting composite being had Osiris's essence, but it took a while for his memories and his full personality to reassert themselves, with the "resurrected Osiris" still acting and speaking like Eliza and possessing her memories also. When he was presented to the Osirians, they perceived Osiris's essence within him, but were reluctant to consider him quite the same person. They dubbed him "Horus", a name which, in their tongue, signified the idea of being "between two spheres". (AUDIO: Words from Nine Divinities)
Civil war against Sutekh
A tribunal was convened to decide whether Horus or Sutekh 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) Horus and Nephthys took their barge to a location underneath the Mediterranean Sea, where they prepared for battle against Sutekh and gathered an army of over seven-hundred Osirian warriors. However, when it became time to strike, Horus led his army to the Homeworld, where they "dealt with" Lolita.
The trap they had devised for Sutekh could only function if Osiris himself died also. To achieve this, after attacking Lolita, Osiris deactivated his biodata from within Horus so that Justine could mortally wound Eliza's body; Eliza then pretended to be Horus in the Temple of Geb and goaded Sutekh into permanently killing her.
Horus and Justine told the official records that Horus and his warriors had vanished while defeating Sutekh; (AUDIO: The Judgment of Sutekh) the Fourth Doctor shared this legend with Sarah Jane Smith, saying, "The wars of the gods entered into mythology. The whole of Egyptian culture is founded upon the Osiran pattern." Sarah recognised the name from the 740 gods recorded in the tomb of Thutmose III. (TV: Pyramids of Mars)
Later rule and death
When preparing to have Eliza get their shared body killed by Sutekh, Horus seemed unconcerned about his own impending demise, noting that it wouldn't be the first time he had died. (AUDIO: The Judgment of Sutekh) Indeed, one account showed that Horus, now in a body with a cow's head, ruled the Osirians in earnest for some time. (PROSE: Scarab of Death) Belying Sutekh's claim that Horus, like the Fourth Doctor, considered all sapient life to be his kith, (TV: Pyramids of Mars) he was a cruel dictator who conquered and oppressed more than a hundred inhabited worlds. Wherever he went, he brought plague and famine; he was known to nurture pacifist societies and then unleash vicious killers on them for his own amusement.
Following the defeat of Sutekh, Horus placed himself in suspended animation in the Black Pyramid on Beta Osiris, Phaester Osiris's MooN. However, the cryogenic systems failed in the 16th century. The Fourth Doctor and Sarah found his corpse in the 2650s. (PROSE: Scarab of Death)
He was worshipped by many cultures. The Egyptians of Earth worshiped him amongst the pantheon of gods, many of whom were named after Osirans. (PROSE: GodEngine) The Cult of the Black Pyramid also worshipped Horus and dismissed Sutekh as a false god. (PROSE: Scarab of Death)
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)
Behind the scenes
Horus is alternately portrayed as both Sutekh's nephew and brother in different sources. While Pyramids of Mars and The Enigma of Sisterhood state they are brothers, The Sands of Time depicts Horus as the son of Osiris, brother of Sutekh. Body Politic reconciled this contradiction by proposing that Horus was a separate entity from, but had the memories of, Sutekh's brother Osiris; in Going Once, Going Twice, Horus refers to Sutekh as his brother, but Sutekh refuses to acknowledge Horus as such.
The ambiguity of the Doctor Who universe characters' relationships reflects that of the real world Egyptian dieties they are based upon, who are alternately described as brothers or uncle and nephew in real world myth (to the extent that some scholars allege the existence of two separate gods by the name of "Horus").