From Tokyopop:

In World of Warcraft: Wrath of the Lich King, Thassarian is a renegade death knight, one of the few of his kind to be free of the Lich King's control. Although Thassarian has turned his incredible powers against his former master, he remains feared and despised by most of his Alliance allies. Countless players have aided Thassarian in game as he battles against the Lich King's agents in Northrend, but few fans know the details of his former life. Death Knight is Thassarian's story, a tale that reveals the origins, motivations, and darkest secrets of Warcraft's newest incarnation of death knights.