Cult of Orrun
The cultists display immense power to warp and control the life-force of the world. Their mages have been cast out as anathema to the other magic-users of the world despite the fact that many of the cultists still fight on humanity's behalf.
The Cult of Orrun have great flexibility in play-style. The ability to reanimate the dead and take control over the living means that a core force of two or three behemoths is just a viable a force as a horde of shambling death. Their play style is chaotic, using magic and manipulation to force other players out of their defined startegies and comfort zones while looking for the opportuniy to beat them into submission.
The Cult is the only faction which can employ a Commander-Sorcerer as well as an additional magic user.