Chaos

Chaos is a mutating and corrupting force of evil which spreads from the Realms of Chaos into the world of Warhammer. Chaos is worshiped by multiple nations and tribes, including the Rotblood Tribe which is encountered in Vermintide 2. Cults exist among humans and elves which secretly pray to the Chaos Gods; cultist in the Empire are hunted by Witch Hunters like Victor Saltzpyre.

Additional information
For more in-dept lore, vist the Warhammer Fantasy entry of Chaos