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 doi