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.

Temporary Invulnerability is mainly a Death effect:

• The Stygian Flesh Bless provides 10 points of Invulnerability to Sacred units; it requires 5 on your Pretender, and that they're on the map.
• 15 points of Invulnerability can be applied with the water of the River Styx. For this, the Stygian Skin spell at Alteration Level 4 lets 11 casters transmute regular water into Stygian Water to apply it to themselves, while the Stygian Rains spell at Evocation Level 7 lets 32 casters pay 2 to make the Styx rain on the entire battlefield.
• 25 points of Invulnerability can be self-applied with the Invulnerability spell, a Level 5 Alteration that requires 3 (or 2 and 1).
• At Construction Level 6, 4 mages can spend 25 to create a Robe of Invulnerability that also provides 25 points.