Arcane Consumption( Player's Handbook II, p. 74)
You can sacrifi ce your physical health to strengthen a spell. This process leaves you wracked with pain, but the enhanced energy you draw from the spell might provide the margin between victory and defeat.
Once per day as a swift action, you can grant the next spell you cast a +4 bonus on its save DC. You must cast and complete this spell on the same turn that you use a swift action to activate this feat. In return, you take a -4 penalty to Constitution for 12 hours and are fatigued.