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The Vacuum Shroom is an item in Mario & Luigi: Bowser's Inside Story. Bowser, after his last defeat, finds a random shop-like area managed by a cloaked figure (later revealed to be Fawful) in the middle of the forest. Here, the figure gives the Vacuum Shroom to Bowser, calling it a "Lucky Shroom" and claims that it will give him the power to beat Mario.
Bowser then uses the Vacuum Shroom, which causes him to mindlessly inhale everything like a powerful vacuum (similar to Bowletta in Mario & Luigi: Superstar Saga, King Boo from Luigi's Mansion, or Kirby and King Dedede from the Kirby series). Following this, he collapses from exhaustion, and the objects he inhales somehow shrink drastically. Bowser then wakes with no recollection of anything that happened after eating it. Later, Bowser gets a controllable, yet less powerful, version of the ability from the Vacuum Block. After Fawful explodes upon defeat, everything Bowser inhaled gets launched back out.
The Vacuum Shroom is similar to the Invincishroom of Mario & Luigi: Superstar Saga; both are mushrooms which are touted to have positive effects, but turn out to be the opposite when consumed by the player character.
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