When you were a child, a parent or other person with authority over you made a minor pact with a fiend on your behalf, stealing some of your vitality in exchange for a minor boon.
Detriment: At the end of any combat, you must attempt a Fortitude saving throw. The DC is 10 + the number of rounds you acted in combat. If you fail this save, you become fatigued for 10 minutes per round you acted during that encounter.