Among the many mortal followers of the Dark Gods, some go the whole hog and sell their souls. Only the strongest and most powerful individuals tend to be chosen, and the gods can rain down blessings on those they favour, up to and including mutations. This has the useful side effect of making them effective Blood Bowl players.
Chaos Warriors are strong, tough and well-armoured, without really compromising their agility. They can be slightly sluggish due to the weight of their armour, though, and tend to rely on Beastmen for their running plays.