Chirurgeon (The Horus Heresy Short Story)

In its earliest days, the III Legion was blighted by a genetic flaw that threatened to end the Phoenician's bloodline forever. Fortunately, through tireless effort and the miracle of the primarch's own genetic perfection, visionaries like Apothecary Fabius were able to halt the corruption and restore the Emperor's Children to their former glory. Except, it would seem, that a terrible secret has been kept from the rest of the Legion, and the threat of further degeneration is quite real... Read it because
Fabius Bile, the notorious and diabolical Space Marine Apothecary, is the epitome of deceit and perversion. In this grim tale, he tries to cure a debilitating sickness in the Emperor's Children Legion. And the secret at the end will startle you just as much as the surgical scenes.