Skeletons

Animated bones and the most infuriatingly common undead foe. Ranges between "class 1" and "class 5", with intelligence and capabilities that increase with class number.

Skeletons are also resistant to most forms of damage (specifically, fire, lightning and cold), and are hard to hit with missiles, due to being bones. They are vulnerable to acid damage. Higher class skeletons can't be damaged by nonmagical weapons.