An Infidel not carrying the Amulet can still cast spells (in contrast to the current setup where they cannot), but instead of casting the spells with Pw, they use blood magic and use HP instead. It doesn’t necessarily cost as much HP as it would Pw - there can be a scaling factor.

If trying to cast a spell that would cost more Pw than the hero has current HP, they die.

Ring or amulet that causes your spellcasting to draw on HP instead of Pw, possibly only when your Pw is zero, or possibly not requiring a ring. Each point of HP is worth 2 Pw points. If cursed, doubles spell Pw costs. If blessed, energy regeneration will regenerate HP until full before regenerating Pw. Casting from HP too much may decrease your HP maximum. If implemented as an artifact, possibly flavor it as an evil counterpart of the Eye of the Aethiopica.

Note that a couple variants have implemented this as a “ring of blood magic” or similar items.