Astraeos was a Librarian of an unknown Space Marine Chapter that in the late 32nd or early 33rd Millennium was targeted for elimination by the Grey Knights under the orders of the Inquisitor Lord Castus Izdubar, following the loss of the Athenaeum of Kallimakus at Apollonia. Astraeos would eventually go on to become a Chaos Space Marine and a potent Chaos Sorcerer as a pupil of the Thousand Sons Exalted Sorcerer Ahzek Ahriman -- and eventually one of Ahriman's greatest foes.
After the Grey Knights assaulted and destroyed his Chapter for unknown reasons, Astraeos and three other members of his Chapter were the only survivors -- its former Chapter Master, Thidias, its Chapter Ancient, Kadin, and another Astartes of the line, Cadar. They secured transport on a rogue Adeptus Mechanicus ship, the Titan's Child, under the captaincy of a rogue Tech-priest, Carmenta, and fled for the Eye of Terror.
After entering the Eye the ailing vessel was captured by the Harrowing, a band of Heretic Astartes Renegades. Cadar was killed in the assault and turned into a Daemonhost by one of the Harrowing's Sorcerers, Maroth, who removed one eye from each of the other three exiles. However, among the members of the Harrowing was the exiled Thousand Sons' former First Captain, Ahzek Ahriman. When an envoy arrived from the Sorcerer Amon seeking Ahriman, Ahriman revealed himself and sought to enlist Astraeos and his fellows as well as Carmenta to be his allies.
Astraeos, not knowing Ahriman's true identity, put his trust in the revealed Sorcerer and became his most devoted pupil, seeking to learn the arts of the Chaos Sorcerer himself. He took his revenge on Maroth by taking both of the eyes of the sorcerer. Astaeos would be troubled by the Daemonhost of Cadar, but chose to not reveal this at first to Thidias and Kadin. Ultimately he bound the daemon to himself. Eventually Thidias was lost and Kadin learned of Cadar's host, but the remaining two exiles continued to support Ahriman.
Lacking a Navigator the Renegades travelled to the border of the Eye of Terror and captured an individual named Silvanus from an Inquisition voidship upon which the Inquisitor Iobel was also a passenger. Though Ahriman was severely wounded in the ensuing battle by the Inquisitor, his plan ultimately succeeded, and Astraeos, Kadin and the daemonhost Cadar helped Ahriman take control of Amon's warband. Despite their initial tensions, the homeless Astraeos and Kadin became Ahriman's most loyal warriors in the years that followed.
Ultimately Astraeos was sent by Ahriman as bait to be captured by the Inquisition so that the Sorcerer Lord might capture Inquisitor Iobel, who he believed knew the location of the Athenaeum of Kallimakus. Astraeos' memories were psychically removed and hidden away, so that his interrogation would consume the time and energies of the Inquisition.
Once in the Inquisition's clutches, Astraeos was tortured, including by a Grey Knights Brother-Captain, Cendrion, but managed to escape his imprisonment during his trial before the arrayed members of the Ordo Malleus. Ahriman had planned for this as well, and struck at the assembled Ordos so as to kidnap Inquisitor Iobel. Whilst the capture of Iobel was carried out successfully, Astraeos was struck down and thought dead by the members of Ahriman's coterie, including Kadin. Kadin subsequently thought himself the last of his Chapter, in the process becoming even more dedicated to Ahriman.
Astraeos, still in captivity, was subsequently blinded, mind-wiped and linked to a Servo-skull so that he could be further interrogated by the Inquisition under Lord Izdabar. He revealed Ahriman's hunt for the Athenaeum when put to the question and the Inquisition's forces then raced Ahriman to Apollonia, where the Athenaeum was located. This was because the Inquisition presumed that just as they had broken Astraeos during interrogation, Ahriman would have broken his captive, Iobel.
At Apollonia, however, a psychic shard of Ahriman's mind that he had implanted within Astraeos' mind provided him with the energy to escape his imprisonment and revealed that it was the Inquisition, and its Chamber Militant the Grey Knights, who had purged his original Chapter. Offered revenge by the fragment of Ahriman, Astraeos took it and went on a blind and psychically-empowered rampage throughout the Inquisition flagship.
When confronted by four Grey Knights, Astraeos was mortally wounded in the subsequent fighting. As he lay dying, he was called upon by the daemon within Cadar's body who he had long ago bound to himself, and was empowered by it. However, Astraeos also realised that his fate, and that of his Chapter, had really been caused by Ahriman.
In aiding Ahriman and in encountering the Grey Knights he had set in motion the demise of his own Chapter, which actually occurred AFTER Ahriman's successful theft of the Athenaeum. In entering the Eye of Terror, Astraeos, Carmenta and his comrades had actually travelled into their own past. With this new knowledge, Astraeos began planning revenge against Ahriman, and stepped into the Warp to escape the flaming ruins of the Inquisition ship.
Eight Terran years after the Battle of Appollonia, Izdubar finally returned to Titan with his surviving contingent of Grey Knights. Izdubar informed Cendrion, now finally conscious after the battle but entombed within the sarcophagus of a Dreadnought, that Ahriman had escaped. Despite this, the Grey Knights had identified Astraeos' Chapter, and Cendrion was now tasked with its destruction. This was an apparent time loop that would lead to the events in Cendrion's future becoming the event in Astraeos's past that caused the latter and his companions to first turn to Chaos.
Some time later at a temple on an unknown planet, two Thousand Sons Sorcerers led by a third named Calitiedies and their warriors met with another Sorcerer in bronze who "...bore the marks of ancient lore on [his] armour, and knew the words of passing from ancient Prospero." The strange, crypto-Prosperine Sorcerer soon revealed himself:
"I am Astraeos, and your oaths will be mine."
- Ahriman: Exile (Novel) by John French
- Ahriman: Sorcerer (Novel) by John French