Dark ztar

he Dark Star is an evil star with dark powers in Mario & Luigi: Bowser's Inside Story.
The Dark Star was originally dug up by Toads long before the events of Mario & Luigi: Bowser's Inside Story. It was put to sleep and sealed below Toad Town by the Star Sprites who feared its power.
Fawful stole the Dark Star after retrieving Princess Peach from Bowser's Body. Fawful then proceeded to head into Peach's Castle and started the process of awakening the Dark Star by transfering Peach's power into it. When Mario and Luigi attempted to get into Peach's Castle, it created a barricade preventing them from getting into the castle. Later, Mario and Luigi used the Miracle Cure to break the barricade and enter the castle.
When Bowser finally got to the room with the Dark Star, Fawful had him fight a stronger version of Midbus called Blizzard Midbus. After Blizzard Midbus was defeated, it broke free of the machine it was held in. Fawful then put on his Headgear and used it to start absorbing the Dark Star's power. Halfway through the process, Bowser punched Fawful while he was transforming into Dark Fawful, sending him flying. Bowser planned to take the Dark Star for himself, but while he was laughing, it flew into his mouth. It then proceeded to enter the Airway in his body and caused Bowser discomfort. Bowser then starts to breathe in cold air emanating from the defeated Blizzard Midbus, allowing Mario and Luigi to access the area. They discover that it is absorbing Bowser's DNA and gaining his traits. When they finally corner it, it fights them.
When it is defeated, it flees from Bowser's Body and turns into Dark Bowser.