Chirurgeon: The Horus Heresy Series (Unabridged‪)‬

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....