Fell Power From Within (3.5e Feat)
From D&D Wiki
Fell Power From Within [General]
Unlike most warlocks, much of your power is drawn from magical stores from deep within your body.
Prerequisite: Eldritch blast ability
Benefit: You now use your Constitution modifier in place of your Charisma modifier when determining the save DCs of your invocations. In addition, when you use an invocation that grants a bonus equal to your Charisma modifier, you instead gain a bonus equal to your Constitution modifier.
Normal: You use your Charisma modifier to determine the save DCs of your invocations and the bonuses granted by certain invocations.