The Boo Mushroom is an avalible mushroom in New Super Mario Bros. 2 (Ult. Avatar). It transforms the user into a boo and they don't get affected by any fellow boos. This mushroom lets you possess any enemy and once you possess them, they will be destroyed. And then some. If you want to beat a boss with this mushroom, you have to possess them 3 times to win.appearances in game Super Mario Galaxy.1.