Not all Dead are reanimated bodies returned from the grave to lay waste to the innocent and pure. Some take whatever form they can, even if it is only part of a whole. When no body can be found, the fell spirits will seek another refuge and finding only an arm, or other body part, they will inhabit the creation and destroy life through this means.
Decayed can be found within unlikely locations, lurking in the crevasses and darkness of stone corridors throughout the barrows. Exposing these creations will require you to spring their trap, but do not be surprised; keep your wits about you, and you will destroy these vile creations.