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.