Spirit shield
Spirit shield is an ego only found on moon troll leather armour, the hat of the Bear Spirit, and some randarts. This ego duplicates the effect of an amulet of guardian spirit, linking your HP and MP pools so that all damage you take will be split proportionally between them. This even affects normally hard-to-resist damage sources such as torment and smiting. Should your MP reach zero, damage will be taken entirely from your HP until at least one MP is restored. HP will not be reduced to 0 before all MP is exhausted.
Deep dwarves gaining spirit shield will have their MP set to zero, and their MP regeneration stops (though, like with HP, other methods of restoring MP still function normally). Djinn do not have MP, so are completely unaffected. Vine Stalkers posess innate spirit shield, so are also unaffected by external sources.
Damage Reduction Example
If you have 200 HP and 20 MP, an 11 damage attack will reduce your HP by 10 and your MP by 1. Characters with more balanced HP:MP ratios will see much more dramatic reductions to HP loss.