The Pile of bones are found in Enakhra's Temple, just south of the Camulet teleport spot. They are level 2 but unattackable. The only option is to climb over them to advance further into the temple.
Prior to the Evolution of Combat update, the Pile of bones used to be a Boneguard blocking the path which required the use of the Crumble undead spell to advance. As the spell was removed, using the spell is no longer necessary. However, the bones left behind remain.