Shrowser is the fusion of Bowser and the spirit of the Elder Princess Shroob. It is the true final boss to be encountered in the game and the eighth boss overall. Mario & Luigi: Partners in Time, though 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 twenty fireballs, ten 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 lose his power. While Bowser is possessed, he looks pretty much the same, the only difference is that he's flashing green and purple and Elder Princess Shroob is floating above him.
HP - 20
POW - 180 (170)
DEF - 999
Speed - 999
EXP - 0
Coins - 0
- Shrowser is the second final boss of a Mario & Luigi game to be a powered-up version of Bowser. The first is Bowletta from Mario & Luigi: Superstar Saga, the third is Dark Bowser from Mario & Luigi: Bowser's Inside Story, and the fourth being Dreamy Bowser from Mario & Luigi: Dream Team.
- Even though Shrowser is indeed the final boss of Mario & Luigi: Partners in Time, many believe that Elder Princess Shroob (transformed) is the final boss due to her difficulty.
- This is the only RPG boss that can be only won by counterattacking with an exception of Thunder Sass from Mario & Luigi: Dream Team. although healing Items are allowed in that battle.
- The purple mushroom that was once the Elder Shroob Princess was not taken out of Baby Luigi's pocket; rather it appears to have come out and navigated into Bowser's mouth by itself. This could mean that the Elder Princess Shroob was not defeated at that point; rather she simply did not have enough power to survive in her regular form. After turning into a mushroom, she decided to use Bowser's body to survive similar to Cackletta's Soul in Superstar Saga.
- The name Shrowser is available only in the game's ROM as well as as in the files as the player cannot attack Shrowser, meaning the player cannot see its name.
- Shrowser is the only final boss in the Mario & Luigi series that doesn't have a spirit that can be fought inside of Bowser's body.
- Shrowser is one of the few foes battled in the present Mushroom Kingdom. The others are the Junior Shrooboid and the Goombas in the sewers.
- Unlike Bowletta and Dark Bowser, who retain the personality and characteristics of their original forms (Cackletta and the Dark Star, respectively), Shrowser still retains the original Bowser's personality, despite being possessed by Elder Princess Shroob's ghost. His pre-battle dialogue even indicates that he may be unaware of Elder Princess Shroob's presence.