Power Shriek

Usage
A Power Shriek is a powerful form of magic that can be performed by a being during a surge of intense emotion. Because the power emanates from within the body of a being, it is a form of magic, but does not belong to the "eight schools" of magic, and is therefore considered to be "the most shameful of all magic".

Usage as an Attack
A Power Shriek, when performed properly, is a scream that reaches such a pitch and volume that it can knock out one's opponent cold. A Power Shriek can be performed without the user knowing of its power, or even if the user is not a magic user, such as Finn, who uses it after a surge of intense anger and jealousy towards Abracadaniel, when he decided that he actually wanted to kiss Princess Bubblegum after Finn forfeits in the Wizard Battle and is turned into a cat by the Grand Master Wizard.

Trivia

 * A Power Shriek can only be performed during a surge of extremely intense emotion, whereas other magic can be called upon using generic commands, making it, though a "shameful" form of magic, an immensely rare one as well.