Cannibalistic Casting

 
Prerequisite(s): None
 
Benefit: You can cast a spell by using a number of constitution points equal to the level of the spell rather than using a spell slot (this feat assumes you have run out of slots). These lost points of constitution are considered ability score damage against your constitution score and are treated as such for all purposes. 0-level spells count as one spell level for the purpose of this feat.Spellcasters that must prepare spells are limited to selecting spells previously prepared and cast that day.
 
Source: Net Book of Feats