Castus Izdubar was a Lord Inquisitor of the Imperial Inquisition in late M32 and early M33. He was Convenor of the Ephisian Conclave and Watcher of the Cadian Marches, directly responsible for the incursions of Chaos in the Cadian Sector. He was also a member of the Ordo Cyclopes, a secret Ordo in the Inquisition dedicated to preserving and protecting the Athenaeum of Kalimakus, the personal remembrancer of Magnus the Red. He became aware of Inquisitor Selandra Iobel's attempts to find out about the Thousand Sons following a psychic cult insurgency on 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 to find out what the Sons of Prospero were doing, in 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, the same Astraeos, was revealed. Ahriman used the gathering -- indeed 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 (members of the surviving Thirteenth Great Company). After losing the Athaenium, Izdabur identified Astraeos of a still-loyal chapter of Space Marines, and had the Grey Knights purge them -- setting in motion Astraeos's flight into the Eye, his travels downtime to encountering and freeing Ahriman, the encounter with Iobel and so on -- creating a loop in which he was doomed to release the Athenaeum again and again.
- Ahriman: Sorcerer (Novel) by John French