Prerequisite: The Outsider patron, Pact of the Tome feature
Your Book of Shadows takes the form of a horrifically stitched together memento of something important in your life. While you hold the Heart, you can use an action and target a creature you can see within 60 feet to gain insight on the target. You can choose to learn the creature's vulnerabilities, resistances, immunities, condition immunities, highest ability score/save, or AC.
Alternatively, the Heart reveals a deep personal secret of the target of the DM's choosing.
This ability can be used a number of times equal to your warlock level before you must complete a long rest.