The bone devil is normally only generated in Gehennom, but is also a valid form for a polymorph trap, which may result in it appearing much earlier. Bone devils are also eligible for gating by other major demons.
Bone devils are not particularly threatening to an average player that has reached Gehennom; as the result of a monster stepping on a polymorph trap, its naturally low AC can make it a potential nasty surprise for players unless the player is sufficiently prepared and has poison resistance. Like most demons, it is vulnerable to silver and cold attacks.
Bone devils attack with weapons and with a great hooked tail
which causes a loss of strength to those they sting.