This is a page on the NPC Demon Skeleton, if you meant the creature, see Demon Skeleton.
During a certain update (pre-2007) the Redbone Castle was changed and a Demon Skeleton which can't be moved or attacked was added. This is an NPC and not a creature because when you look at him the message reads "You see Demon Skeleton." and not "You see a demon skeleton." which would happen when it had been a creature. Also, when trying to attacking him, the message "You may not attack this person." will appear.