Shrowser is a combination of Bowser and the spirit of the Elder Princess Shroob. It is the final boss to be encountered in the game Mario and Luigi: Partners in Time; oddly, the boss battle is similar to 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 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.
- In all three Mario and Luigi games, Bowser has been fused with the spirit of a main villain. In Mario and Luigi: Superstar Saga, Cackletta became Bowletta, in Mario and Luigi: Partners in Time, Elder Princess Shroob became Shrowser, and in Mario and Luigi: Bowser's Inside Story, Dark Star became Dark Bowser.
- The floating spirit of Elder Princess Shroob above Shrowser points with her right or left tentacle multiple times before launching her fireballs and fire orbs. This tentacle pointing shows in what order she will attack the Brothers and their baby counterparts; the right tentacle referring to Mario and/or Baby Mario, and the left tentacle referring to Luigi and/or Baby Luigi.
- 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 possess Bowser and use his form to survive.
- Elder Princess Shroob is in her transformed form while being above Shrowser's head.
- The name Shrowser is available only in the game's ROM as the player cannot attack Shrowser, meaning the player cannot see his name.