Blood of Martyrs is a religion sourcebook for Dark Heresy. Of course, since this is Warhammer 40k, this means talk of the One True Religion (worship of the Emperor), with all else being heresy. On the other hand, having One True Faith typically means that you’ll have lots of factions, cults and interpretations of that faith… and that’s where this book steps in.
We get a lot of detail on the internal workings of the Ecclesiarchy, including details on many notable and/or important NPCs, all of which is great stuff for players of church-oriented characters, especially in campaigns featuring internal schisms and political intrigue. The church of the God-Emperor of Mankind is a vast and fairly fragmented construct, even though it tries to maintain a unified official front for the masses. Historical corollaries can be drawn with the Catholic Church in medieval and later times in Europe: religion is politics and vice versa.
Additionally, the book contains multiple new options for Adepta Sororitas characters. In addition to the basic “Sister of Battle” template given in the Inquisitor’s Handbook, we now get interesting variants into other directions: Sisters specializing in social and genetic control (shades of Bene Gesserit here), and others specializing in information gathering. Not to mention specialized healer/medic options.
Also included are a bunch of new church-oriented career paths. Confessors, Drill Abbots, Witch Finders, etc. Should be useful in creating religious PCs, should the options given in the core book seem limited. All in all, it’s a nice package of church and religion -oriented background information in the Warhammer 40k world, and as always the production values are top notch.