Fossilize

This spell permanently hardens the bones of one skeletal undead creature, making the bones denser and stronger. The fossilized skeleton permanently gets +2 on damage rolls, +4 on its AC, and its damage resistance is enhanced. A fossilized skeleton takes only quarter- damage from edged weapons, and half-damage from any other weapons. The fossilized skeleton becomes immune to normal arrows, bolts, and bullets. Magical missile weapons do only their 'plus' in damage. While this spell is permanent, there is a drawback. The newly-fossilized skeleton may not be repaired or otherwise 'healed', as the organic materials that made up the bones has been completely replaced by minerals. Intelligent skeletal undead would not normally want this spell applied to themselves, but the spell would affect them nevertheless.