A slow, stupid creature that crawls onto people and latches onto them, often on feet, hands, and other extremities, numbing down the area and slowly feeding on it. It's easy to avoid while awake, but nearly impossible to remove once attached. Many an adventurer has been forced to hack off an arm or leg after waking with a Stoneleech wrapped around it. Sleep with one eye open! (This idea came from "Wizard's Rule")
Thunder and lightning is the sign of their shamans hunting and killing demons. Children's souls must be protected for their souls are not firmly in place yet and are easy for demons to take hold of. The shamans must hunt and destroy them and each break of thunder is another death of a demon hunting for souls.