John French
Ahriman: Sorcerer
Ahriman (series) part 3

Ahriman, greatest sorcerer of the Thousand Sons and architect of the Rubric that laid his Legion low, continues to walk the path towards salvation, or damnation. Searching for a cure for his Legion, he is forced to consider – was the great ritual somehow flawed from the very beginning?
Warhammer 40k
novel

Also in:
Ahriman Collection
Ahriman: the Omnibus
Eye of Terror