Gregor Eisenhorn is a Inquisitor in the Warhammer 40,000 universe, serving the God-Emperor and protecting his people from the machinations of the fiends that serve chaos, who are always hovering, trying to worm their way into the Empire, even through those who are supposed to serve the Emperor.
Eisenhorn's battles with the ultimate Evils of Chaos and the warp serve as the subjects for three major books (Separately published under the titles of Xenos, Malleus and Hereticus), and two short linking stories that tie the three tales together. In them, Gregor goes from a young and eager Inquisitor who holds himself to the highest, most scrupulous standards of behavior, to someone who is far less scrupulous and aboveboard in his methods of enforcing orthodoxy and overcoming chaos and evil, while still remaining an effective Inquisitor.
Of course, he cannot do it alone. Gregor travels with an entire group of characters who provide him with help and support, from Midas Betancore who acts as his personal pilot, to Aemos, who is a researcher who comes close to being a human data retrieval machine. Along the way, he picks up and loses help at a phenomenal rate, with two female combat experts being blown away from under him right at the beginning of both Xenos and Malleus. The second, Arianrhod, leaves behind her family's saber, Barbarisater, to remember her by, and it is this weapon that is instrumental in the downfall of the Chaos-creature Cherubael later on in the book.
Eisenhorn also takes a great deal of toll on his friends and allies. Ravenor, his student, goes from being a strong and vigorous young man to being paralyzed and disfigured, but went from a secondary character to the main character in another series also written by Dan Abnett.
I love Dan Abnett as a writer. He makes you feel both the wonder and the essential bleakness of the Warhammer future in his writing, but unlike some of the other authors who write books for the "Black Library", he also makes you feel for his characters. In a less-talented writer, Eisenhorn could come off as a cold character who you wouldn't care about at all. But Abnett writes him as a very human character for all his cold attitudes and training, one whom you end up caring about deeply.
Indeed, it's not just Eisenhorn who comes in for this sort of treatment, but all of Dan Abnett's characters, from Eisenhorn to Ravenor, to Ibram Gaunt, or Gaunt's Ghosts.
Dan Abnett remains my favorite writer of all those in the "Black Library", and this huge three-in-one volume provides ample reason as to why.