The Dark Star Core (also known as Dark Fawful) is the fusion of the Dark Star and Fawful, and serves as the final boss for Mario and Luigi in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. It consists of Dark Fawful's remains and the Dark Star. In order for Mario and Luigi to attack the Dark Star Core, Bowser has to knock out Dark Bowser first. After that, the Dark Star Core will revive Dark Bowser. And after Dark Bowser has been revived, Bowser has to punch his stomach, forcing Dark Bowser to spit out the Dark Star Core. Bowser then uses Vacuum to suck up the Dark Star Core. Then Mario and Luigi can attack. Mario and Luigi must destroy Fawful so they can defeat the Dark Star once and for all. After a few turns, Dark Star will resurrect Fawful and leaves Bowser's stomach, forcing Bowser to spit the Dark Star Core into Dark Bowser, meaning the player must do the phase
all over again. After defeating the Dark Star Core, Bowser can do the final punches to Dark Bowser. Then, Fawful explodes inside Bowser's stomach and expels everyone out of Bowser's body and negativing the effects of the Vacuum Mushroom. Then Bowser figures out that Mario, Luigi, Starlow, and the others were in his body. Enraged, Bowser battles Mario, Luigi and Starlow, however, the Mario Bros. managed to defeat Bowser. After the Dark Star Core exploded, it was gone forever.
- When one of Fawful's eyes are damaged, the Dark Star Core closes one of its eyes as well. This strongly implies that Fawful might be controlling the Dark Star and thus Dark Bowser, an idea further supported by the fact that Fawful tries to flee back into Dark Bowser's body when Bowser inhales him.
- The Dark Star Core is the second final boss to be fought inside Bowser's body. The first was Cackletta, Fawful's own master.
- The Dark Star Core is the third final boss of the Mario and Luigi series that has an attack that involves swinging an object around in circles. The other two were Cackletta's Soul and Elder Princess Shroob.
- The battle background is only seen in this battle even if the bros. are in an area where they had previously fought.
- Aditionally regardless to what area in Bowser's Body the player were in before the fight, the background in the cutscene as well as the fight are always the same.
- The Antenna attached to Dark Fawful's head with the Dark Star is similar to the antenna on his head in the final battle with him in Superstar Saga and in Partners in Time.