From the Super Mario Wiki
The Vacuum Shroom is a mushroom item in Mario & Luigi: Bowser's Inside Story and its 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. 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 Dimble Wood. 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.
When Bowser eats the Vacuum Shroom, which causes him to mindlessly inhale everything like a powerful vacuum (similar to Bowletta in Mario & Luigi: Superstar Saga and Mario & Luigi: Superstar Saga + Bowser's Minions, King Boo from Luigi's Mansion, or Kirby and King Dedede from the Kirby series), including Mario, Luigi, Princess Peach, Toadsworth, Starlow, Toadbert and many Toads in Peach's Castle. 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, given by Broque Monsieur after bringing the island he was stranded on back to shore in Plack Beach. The block allows Bowser to inhale items from enemies to recover HP or gain a boost in stats, or inhale smaller enemies like Coalbits and Sneeds for Mario and Luigi to battle. The last function of the Vacuum Block is featured heavily in some boss fights, like the Sea Pipe Statue, Blizzard Midbus and Dark Bowser.
After Fawful explodes upon the defeat of the Dark Star Core and ultimately, Dark Bowser, everything Bowser inhaled gets launched back out by a weakened Dark Fawful, who exploded inside his body in a last ditch effort to destroy Mario and Luigi.
In Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, the Bowser Jr.'s Journey side-story implies that a side effect of the Vacuum Shroom's effects wearing off from Fawful's defeat is that it left Bowser somewhat bloated, as Bowser, when learning from Bowser Jr. that he got the Skeletone Formula:D, and was offered some of the formula, he accepted, citing he was still a bit bloated from the Vacuum Shroom's effects, only to ingest way too much of the formula and caused him to lose too much of his mass. In addition, the remake also revealed that Dieter of the Best Fitness Friends was the one responsible for providing Fawful with the mushroom in the first place.
The Vacuum Shroom is similar to the Invincishroom/Poison Mushroom mistaken as a Invincibility Mushroom of Mario & Luigi: Superstar Saga and Mario & Luigi: Superstar Saga + Bowser's Minions; 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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