Dark One's Own Luck

Least Invocation; 2nd
You are favored by the dark powers if you have this invocation. You gain a luck bonus equal to your Charisma bonus (if any) on Fortitude saves, Reflex saves, or Will saves (your choice each time you use this ability) for a period of 24 hours. You can’t apply this ability to two different save types at the same time. This bonus can never exceed your class level.

Source: Complete Arcane