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Shrowser is the form that Bowser takes after ingesting the mushroom of the Elder Princess Shroob. He is the final boss to be encountered in Mario & Luigi: Partners in Time, though the boss battle is more like a minigame. After Bowser ate the purple mushroom that held the essence of Elder Princess Shroob, he got possessed by her. Mario, Luigi, and their baby forms had to fight this new threat in Peach's Castle.
This fight is unique because, as Shrowser says, the player cannot perform any actions, such as attacking or using items. In order to defeat Shrowser, the player must Jump over or hit back with the Hammer 20 fireballs, 10 fire orbs, or a combination of both, in order to destroy purple mushrooms, which indicate Shrowser's life. After destroying all mushrooms, Elder Princess Shroob will disappear, causing Bowser to faint.
While Bowser is possessed, he retains his normal appearance and personality, although he flashes purple, has purple clouds around him, purple eyes and an afterimage effect (probably showing his high speed, thus explaining why Mario and Luigi cannot attack).
The music during the Shrowser battle is the same as during the earlier battle against Bowser with his baby counterpart.
Shrowser's attacks can be countered by looking at the Elder Princess Shroob. She will point with her left or right hand. If she points with her right hand, Shrowser will attack Mario and Baby Mario. If she points with the left hand, Shrowser will attack Luigi and Baby Luigi.
Names in other languages
Because the brothers are unable to attack, the name "Shrowser" never appears in-game. However, it does appear as a string in the ROM. This string is translated in some releases of the game, revealing probable names for Shrowser in those languages.