Castus Izdubar was an Inquisitor Lord of the Imperial Inquisition in the late 32nd and early 33rd Millennia. He was Convenor of the Ephisian Conclave and Watcher of the Cadian Marches, directly responsible for the incursions of Chaos into the Cadian Sector.
He was also a member of the Ordo Cyclopes, a secret minor ordo of the Inquisition dedicated to preserving and protecting the Athenaeum of Kallimakus, a great library of forgotten knowledge buried beneath the world of Appollonia, maintained by a cult of savants that worshiped the memory and works of Mahavastu Kallimakus, the Remembrancer who served as the personal scribe of Magnus the Red, Primarch of the Thousand Sons Legion, during the Great Crusade.
He became aware of Inquisitor Selandra Iobel's attempts to discover information about the Thousand Sons following a psychic cult insurgency on the world of Carsona, and attracted her to AV-213 in the Halo Margins. There he introduced her to the existence of the Thousand Sons and the Ordo Cyclopes.
Ultimately, Iobel was sent on a mission into the Eye of Terror to find out what the sons of Prospero were doing, during which her ship was assaulted by three Renegade Space Marines -- including Ahriman and Astraeos.
After her return, a gathering of the ordos occurred in which a captured Sorcerer-Astartes, Astraeos, was revealed. Ahriman used the gathering -- indeed had orchestrated it -- to kidnap Iobel. He succeeded, and a chase began to reach the Athaenaeum at its bastion on Apollonia.
There a battle occurred between Izdabur's forces, Ahriman's fleet and a rogue fleet of Space Wolves vessels who had chased Ahriman out of the Eye. All were members of the surviving Thirteenth Great Company.
After losing the Athenaeum, Izdabur identified Astraeos as a member of a still-loyal Chapter of Space Marines, and had the Grey Knights purge them -- setting in motion Astraeos' flight into the Eye, his travels into the future to encounter and free Ahriman, the encounter with Iobel and so on -- creating a temporal loop in which he was doomed to release the Athenaeum again and again.
- Ahriman: Sorcerer (Novel) by John French
- Codex: Chaos Space Marines (6th Edition), pg. 60