The Decayed are a type of Barrow-wight, or at least what is left of them. This category includes crawling severed hands that move under their own power. They are found in tombs of the Barrow-downs and Imlad Balchorth and other places. They are Swarm and move very slowly; they are always aggressive. They are weak to Westernesse damage and resistant to Common and Physical.
A list of all known sightings: