Dark Vaettirs are new enemies introduced in Conquest mode of the game.
In Norse Mythology, Vaettir refers to a group of faeries/spirits, and Dark Vaettir is one of such spirits. Unlike the others, Dark Vaettir are inherently hostile beings native at Niflheim. Using rocks and ice as the medium, they manifest in material world as brutish, horned golem-like creatures with brute strength and durability.
A number of Dark Vaettir appeared as bodyguards of Valdemar The Cursed, as long as they are around, Valdemar cannot be killed again. The reason of this is possibly the result of a pact Valdemar made with these creatures so he can return as a more powerful Draug.