Soul Sacrifice

 
Prerequisite(s): Cha 15+, Cast Arcane spells without preparation, Toughness
 
Benefit: An arcane caster that does not need to prepare spells can sacrifice hit points and gain more spell slots. The cost is one hit point per spell level. Cantrips cost 1 hit point. The loss is permanent, the spell slots are not.
 
Source: Mongoose Ultimate Feats