Magic and it's costs.

I always wanted to make a spell caster who sacrifices part of themselves for their magic. Usually by the way of making their casting cost them Hit Points or some such. Normally I referred to them as Blood Mages or Life Magic, not really sure on a good proper term for them.

Suggested it as an alternate system for the old boxed set D&D games I ran. It go howled down because they have so few hit points even though it would allow them to cast more than one spell during the game. Never bought the idea of you memorized it and then forget it when cast, the spells are a mages life, they never ever forget them. Well that's my view. So this also could be done a few other ways. Maybe make it pain magic and they actually have to injure themselves to cast it.

Anyways just my thoughts at the moment again.