Skeletons are undead monsters found inside the Barrows crypts. They were presumably reanimated by the Zarosian Mahjarrat Sliske in order to protect his Barrows wights and their treasure. They are aggressive, and like most skeletons they are weak to the Salve amulet and Earth spells. Like the other monsters in the Crypts, they can be killed to increase your Barrows killcount, which increases the chances of getting Barrows equipment from the chest, although the effect seems to cap at around 8 killed.