Invulnerability represents certain creatures' resistance to nonmagical weapons; it is often found on highly magical creatures like angels (Harbinger), and can be granted with the spells Stygian Skin and Invulnerability.

A creature's invulnerability value will replace its natural protection value against nonmagical attacks if it is higher. If a creature's invulnerability value is less than its natural protection, it will have no effect.

It is a useful ability for thugs that want to survive against swarms of mundane troops, but does nothing to protect them from counterthugs or elites with magic weapons.