“Mmm... I can feel a power burning in my belly! What in the world did you feed me?”
Shrowser is the form that Bowser takes after ingesting the spirit of the Elder Princess Shroob. He is the final boss to be encountered in the game Mario & Luigi: Partners in Time; oddly, the boss battle is more like a mini-game. After Bowser ate the purple mushroom that held the soul of Elder Princess Shroob, he got possessed by her. Mario, Luigi, and their baby forms had to fight this new threat in Princess Peach's Castle.
This fight is unique because, as Shrowser says, the player cannot attack. 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. On a side note, the bros. cannot heal either, so they must be careful not to take damage in the fight.
While Bowser is possessed, he retains his normal appearance (for the most part) 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 can't 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 the two Marios. If she points with the left hand, Shrowser will attack the two Luigis.
 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.