Spellburn.
"Blood aids sorcery."
From Dungeon Crawl Classics RPG from Goodman Games.
A magic-user sacrifices part of his soul, some of his strength, or life energy to cast a spell in an emergency.
It has some potential in LL. Possibly give up HP or points in a stat to cast a spell. One point per spell level. Maybe make a save or the loss (or part of it) is permanent.
That could make sure it is only used only in emergencies.