Black Crusade (RPG)
This is for the RPG, not the apocalypse-style crusades by Chaos.
Black Crusade is the fourth Fantasy Flight Games Warhammer 40,000 Role-Playing Game, and its main focus is the forces of Chaos, like cultists and the Traitor Legions.
This is the exact opposite of the earlier books in every way. Even the Human characters in Black Crusade are considerably more powerful than those from previous games, able to receive powerful mutations and game-changing blessings. The Chaos Space Marines are even more powerful, having the benefits available to human heretics as well as the hard combat capabilities of space marines, and weaponry ranges from rune swords, to daemonic bolters, to reaper autocannons. Alignment often leads to extremely interesting role-playing as well.
One of its greatest features is encouraging competition between players, often by making the personal goals of their characters mutually exclusive, or counterproductive to the mission, thus mirroring the climate of a Chaos warband. This can go very right, or oh so wrong.
NOTICE:: As of 9/9/16, FFG announced that the contract with GW has "expired" and they will no longer be producing anymore 40K or WFB products. We've got until Feb. 28 of '17 to stock up, and then all of their products with GW's IP will be removed from the catalogues. It seems that the 40K RPG lines are currently and officially dead until/unless GW finds a new contract to carry on the legacy or builds an in-house team to do it.