John French
Ahriman: Unchanged
Ahriman (series) part 3

It has taken many long years and countless sacrifices, but finally Ahriman, former Chief Librarian of the Thousand Sons, now exile and sorcerer, is ready to attempt the most audacious and daring feat of his long life.
Warhammer 40k
novel

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