Oh, spiritual resistance is involved. I looked up the skill online (thinking that would be faster than starting the game and loading a save) and it wasn't mentioned, where IIRC that site usually does mention that where applicable.
Spiritual resistance is kind of similar to lightning, fire and poison resistance, but it is more for determining if certain spells (like Polymorph, etc) will effect the target, or not, rather than mitigating specific types of damage. For example, I mentioned the survivor's Curse spell above, since the mage's Withering Curse spell can be resisted with a high enough spiritual resistance; since the survivor's version can not be resisted, it is much more effective.
