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 eats the purple mushroom that held the essence of Elder Princess Shroob, he is possessed by her, and Mario, Luigi, Baby Mario, and Baby Luigi fight them 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 Hammer back 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 with multiple signs of Elder Princess Shroob's influence; he flashes purple, has purple eyes, and has an afterimage effect and purple clouds around him.
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 and her attack patterns. If she points with her right arm, Shrowser attacks Mario and Baby Mario. If she points with the left arm, Shrowser attacks 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.