From page 107 of the Player's Handbook (5e, 2014):
At 3rd level, your otherworldly patron bestows a gift upon you for your loyal service. You gain one of the following features of your choice.
Here's a new variant for the ancient eldritch power of intent:
Pact of the Intent
Whenever an action you take has what you consider to be an unintended outcome, you can roll a d20 and add it to your attack roll or subtract it from your opponent's saving throw. You can do this as many times as you like.
Additionally, if your critical fumble hits a teammate and causes them to faint, you get the XP from the monster you were targeting - it's only fair. You get this XP even if your attack would not have killed the monster.