Apep

Apep was a monstrous snake created by the Great Houses as a weapon against the Osirians.

Creation
When they discovered the existence of the Osirian Court, the Great Houses of the Homeworld became frightened of their power to extinguish suns, given that the Homeworld drew the better part of its power from an engineered sun. Given that the Osirians' Ship of a Billion Years also had an engineered sun at its heart, Ra, the Houses decide to lock themselves and the Osirians in a state of mutually assured destruction by inventing a weapon that could threaten the Osirians' sun just as much as the Osirians' power threatened the Homeworld's sun. This weapon was a gigantic, sapient, murderous serpent with bones made of stone and who could devour a sun whole: Apep. (PROSE: Going Once, Going Twice)

Unanchored
However, the Osirians managed to capture Apep and free him from the Anchoring by inoculating him with a battery of alter-time states, including anti-matter, anti-time and the Schrödinger Virus. They hoped to turn the Houses' weapon against them, making Apep a guardian of Ra rather than a threat to it. However, Apep went insane and was deemed too dangerous to be let anywhere near Ra. He was consequently locked away in a sarcophagus, nearly dead. (PROSE: Going Once, Going Twice)

Fate of Apep's sarcophagus
In the civil war between Horus and Sutekh that tore apart the Osirian Court, the Ship of a Billion Years was destroyed and Apep's sarcophagus was lost. It ended up at the Fields of Cratosi, where Faction Paradox sent a battalion of skulltroopers (including Cousin Intrepid) to recover it in the hope that it could be used in an assault on the Citadel of the Homeworld. The Great Houses got involved, turning the mission into a battle that ended with all the combattants frozen in Time. (PROSE: Going Once, Going Twice)

Auteur's scheme
Some time later, in the days of the Armistice, the sarcophagus ended up as one of the items auctioned by Cortalian at the First Auction in Heaven. Auteur convinced Mother and Father that Apep could be used to return the Family of the Shadow Spire, their splinter-group of the Faction, to power, and they obligingly sent Cousin Gustav and Little Brother Kifah to the Auction to acquire the sarcophagus.

Once it was brought to him, Auteur opened the sarcophagus and invoked the loas so that they'd help him bond his biodata to Apep, by which means he intended to free himself from the Spire and become the uncontested master of the Spiral Politic. However, Apep used his unanchored nature to instead snake backwards up Auteur's timeline and subvert the narrative he had been weaving, to instead culminate in Auteur's death and Apep's true resurrection; Apep sucked Auteur's shadow-skin away from him, causing him to begin to disintegrate, and then swallowed him whole, feeding his biodata to the loas to make them loyal to him. (PROSE: Going Once, Going Twice)

Final defeat
Apep's arising caused the immediate past and present to begin unraveling, and he materialised over the First Auction in Heaven at full power. The Battle of Cratosi Fields was unfrozen, freeing Intrepid, who was reunited with Kifah and Gustav. Realising that the only reason the Great House regen-inf soldiers' weapons were failing to harm Apep was that he was protected by the Loas, Gustav and Kifah decided to distract them by ripping apart the body of the dying Cortalian while Gustav focused on his belief in the reality of his species' history. Distracted by these alternative sources of biodata, the loas flocked away from Apep, leaving him exposed, and began falling through the very cracks in Time that Apep had created.

Weakened and injured, Apep was reduced to a patch of Unanchored time. Tauntingly, Kifah observed that this meant he was only real if a Homeworlder dared to use the Observer Effect on him — and then covered Intrepid's eyes, stripping Apep of his last tether to reality. While the two made their escape, Apep crashed to the ground, bleeding tar, moments before the whole planet Kratoam ceased to exist completely, all of its reality falling through the cracks in Time that Apep had opened. (PROSE: Going Once, Going Twice)

Behind the scenes
In Egyptian mythology, Apep or Apophis was a giant snake with a head made of flint. As well as an eternal nemesis of the Sun God Ra, who tried to eat him every night as he passed through the Underworld, Apep was also an embodiment of chaos, opposed to "Ma'at", a complex and sometimes-personified concept in the Ancient Egyptian religion, which could be translated as "truth" or as "order". The latter meaning corresponds best to Apep's depiction in Faction Paradox as an "unanchored" being not bound by the laws of rationality and science edicted by the Great Houses via the Anchoring.