Mario & Luigi: Partners in TimeMario & Luigi: An Ultimate Crossover
Princess Shroob is the main antagonist of the game Mario & Luigi: Partners in time. She is second in command of the alien race known as the Shroob, although initially it appears that she is their leader. She set off a full invasion of the Mushroom Kingdom to make it their own. She had invaded the mushroom Kingdom once again in Super Mario Universal Mission.
Princess Shroob had invaded the Mushroom Kingdom along with her sister Elder Princess Shroob because Planet Shroob was withering. After the Shroobs manage to get inside Princess Peach's Castle, they find Princess Peach, who had traveled back in time with E. Gadd's Time Machine. They attempt to capture her, but before they can, Princess Peach uses the Cobalt Star to imprison Elder Princess Shroob and scatters the shards of the Cobalt Star. The younger Princess Shroob manages to capture Peach.
In Baby Bowser's Koopa Cruiser, Princess Shroob is seen in a transmission, however she remains hidden in the shadows. From Shroob Castle, she fires a missile, aiming for the Koopa Cruiser. Since the Koopa Cruiser was low on ammo, the missile hits the airship and it crashes in Baby Bowser's Castle.
Later on, in the Koopaseum, Princess Shroob appears on stage to celebrate the conquest of the Mushroom Kingdom by feeding Princess Peach to Petey Piranha. However, Mario, Luigi, Baby Mario, and Baby Luigi interrupt her by getting onstage. Princess Shroob then orders the Shrooboid Brat to defeat them. Mario and company eventually defeat the Shrooboid Brat, but after the fight, Princess Shroob breaks a hole under the heroes, and they fall to the underground.
After falling underground, Mario and company set out to rescue Peach in Gritzy Caves. When they reach her, she has a Shroob Mushroom attached to her face. They take her back to Princess Peach's Castle in the future. This princess was actually Princess Shroob, and fools everyone, even Bowser, who kidnaps her believing she is Princess Peach. Bowser brings her to Thwomp Volcano, and after he is defeated by Mario and company, the Shroob Mothership arrives and captures the princess. Mario, Luigi, and their younger selves follow, still believing she is Princess Peach. Once they finally reach "Princess Peach", Princess Shroob reveals herself. She is about to get rid of them, but is blasted away by a disguised Kylie Koopa. This victory is short lived when she recovers and then drops Mario and his friends off of the Shroob Mothership.
The four Mario bros. eventually make their way through a shroobified Peach's Castle and hijack a Shroob Battleship and engage the Shroob Princess in a dogfight with the Shroob Mothership. After blasting the Shroob Mothership out of the sky the Princess emerges from the wreckage. She appears to have gone mad, declaring Peach useless, the kingdom hers, and that she will destroy them all, shouting "Destroy!" several times. She then enters battle with them.
The Cobalt Star shards are located in many places that Princess Shroob should have been aware of, such as the Vim Factory. It is unknown why she did not attempt to have the Cobalt Star restored and bring her sister back to power.