Bones of Steel

This spell greatly strengthens the bones of animated skeletons, literally making them as hard as steel and able to resist far greater amounts of damage.

Casting bones of steel on a skeleton will grant a +2 bonus to its Armor Class. The spell will stay in effect until the skeleton is destroyed or be dispelled.

Regardless of the number of times this spell is cast, you may only strengthen the bones simultaneously in a number of skeletons equal to your spellcasting modifier.