David Annandale
Imperial Creed

As war engulfs Mistral, a plot unravels that pits old friends against one another. Chaos cults, Adepta Sororitas and inquisitors all stand between Yarrick and his mission. Here is where the legend began. In this crucible was Lord Commissar Sebastian Yarrick forged in blood.
Warhammer 40k
novel

Also in:
Yarrick: the Omnibus