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Invulnerability represents certain creatures' resistance to nonmagical weapons; it is often found on highly magical creatures like liches (Lich) and 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.