The Dark Star is an evil star with dark powers in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. It is said that if a person with an evil heart were to awaken the Dark Star and unleashes it's power, that entity would gain enormous abilities.
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 transferring 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 helmet 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 the Dark Star is defeated, it flees from Bowser's Body and turns into Dark Bowser. Dark Bowser realizes its power is incomplete and kidnaps Princess Peach, to no effect. Then it started to search for Fawful, who has the other half of its power.
Bowser eventually fights Dark Fawful, whose abilities have been increased after absorbing half of the Dark Star's energy. Afterwards, he turns into a bug-like creature and scuttles off. Then Fawful gets absorbed by Dark Bowser, and Dark Bowser gains both halves of the dark power. He then heads for the top of the castle, and unleashes a dark hurricane.
Bowser fights Dark Bowser at the top of the castle, and knocks Fawful out of his stomach. He then eats Fawful, who uses his Dark Star Core to grow in size. Then Mario and Luigi attack the Dark Star Core at the top of Fawful's head, and destroy it, crippling Dark Bowser. Bowser then punches Dark Bowser off of Peach's Castle therefore is defeated. After the battle, Dark Bowser turns back into the Dark Star, and explodes, causing the storm to end.
Fawful, who is still in Bowser's body, explodes in a last-ditch attempt to defeat the Mario bros. Instead, the Mario Bros., Starlow, and the Toads inside Bowser's body get sent out of the body, and all traces of the Dark Star are gone.
- Dark Star's beam attack is similar to R.O.B.'s Final Smash.
- The Dark star is the only main antagonist in the Mario & Luigi games that did not possess or otherwise take control of Bowser (a feat which Fawful had briefly pulled off early in the game with the vacuum shroom); instead, the Dark Star "copied" him and became a clone of Bowser.
- The Dark Star's huge laser attack is the only one of the bottom screen filling attacks that Mario and Luigi need to grab on to Starlow to avoid in the game that has two parts. It also is the only one where they cannot hang on to Starlow as long as they want after the attack is done, because the Dark Star will ram them a few instants after his laser is shot. The Dark Star X does the same thing.
- This (alongside Swiggler and Sunnycide) are the only non-final bosses in the Mario RPG series to have complete invincibility, making it only possible with a complicated strategy although Swiggler and Sunnycide didn't have complete invincibility they just had super high defense when in good condition and this can easily be changed if overleveled unlike the Dark Star.