The Soul Cairn is a mystical location on a plane of Oblivion where disrupted or otherwise cursed souls often find themselves trapped. They may have been cursed by a magically powerful being, or indeed a daedra lord, or sent there after being disrupted in the process of being trapped in a Soul Gem and their energy thereafter consumed in its use. The latter possibility is offered by arcane theorists who insist that souls trapped in Soul Gems that are later expended are never truly destroyed.
The Soul Cairn is defended by undead who are controlled by elder spirit beings known as the Ideal Masters. These undead, similar to draugr, are known as Bonemen. There are several giant-sized, Dragonplate Armor-plated warriors overlooking the Cairn known as Boneyard Keepers. When any of these undead are slain, all that remains seems to be a black, liquid puddle.
Also inhabiting the Soul Cairn are the countless souls that arrive there and remain, apparently forever trapped.