From the Super Mario Wiki
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 gives him the ability to 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). Using the ability makes Bowser feel tired, and objects he inhales somehow shrink drastically. It was presumed destroyed after Fawful is defeated: however, Mario & Luigi: Dream Team reveals that Bowser is still capable of inhaling in that game, implying that the Vacuum Shroom did not perish with its creator, or that inhaling became a "naturalized" ability for him.
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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