Cyrene was once an Imperial Civilised World of Sub-sector Aurelia, in the Korianis Sector of the Segmentum Ultima. The planet was used as a source of recruits for the Blood Ravens Chapter of Space Marines. Shortly before the events on the world of Tartarus in Dawn of War, Cyrene was subjected to an Exterminatus action by order of the commander of the Blood Ravens' 3rd Company, Captain Gabriel Angelos, after the seed of heresy was discovered within the planet's population.
Angelos had been on Cyrene to conduct his Chapter's Blood Trials which were intended to identify and induct new Aspirants into the Blood Ravens. But in the course of the trials Angelos discovered evidence of wide-spread heresy and mutation amongst the populace of his homeworld. In truth, Cyrene was not afflicted by the usual forms of heresy seen across the Imperium but by an unexpected cultural change.
Cyrene was ready to throw off the shackles of the Imperial Creed and embrace forbidden ideas such as planetary democracy, free trade with xenos species, and allowing psykers to practice their powers unrestricted while living among the population without stigma or being handed over to the Black Ships.
Horrified at this turn of events, Angelos quickly sent a coded message to the Inquisition, who performed an Exterminatus action on the world. This was an extraordinarily difficult decision for Angelos, who had called Cyrene his own home before he was recruited by the Blood Ravens.
During the Third Aurelian Crusade, the heretical Chapter Master of the Blood Ravens, Azariah Kyras, came to the blasted surface of Cyrene with his followers in order to conduct his apotheosis to Daemon Prince rank without interruption by the unsuspecting Imperials. His plan failed, and the factions fighting in the sub-sector followed him.
Cyrene became the final battleground for the Third Aurelian Crusade, where Kyras met his end as a half-ascended Daemon Prince of Khorne. The planet is now an S-Class Dead World and all further travel to the world is forbidden by order of the Inquisition.