Dark Star Core

Dark Star Core is the final boss of Mario & Luigi: Bowser's Inside Story, along with Dark Bowser. The Dark Star Core consists of the remains of Dark Fawful after Bowser weakened him combined with the weak point of the Dark Star. In this form, Fawful has greater powers, along with slightly altered appearances, such as red glass eyes, changeable appendages, and the Dark Star attached to his head.

Mario & Luigi: Bowser's Inside Story
In the final fight of the game, Bowser has to make Dark Bowser collapse by making him lose 1000 HP. Dark Fawful will then recover him and make Dark Bowser giant. Bowser then has to attack the giant Dark Bowser's stomach to make him spit out the Dark Fawful. Bowser then has to inhale Dark Fawful and let Mario and Luigi battle him. When in Bowser's body, Dark Fawful will become giant sized.

Dark Fawful's second form consists of 6 parts: His two glasses, three appendages, and the Dark Star Core located on his head. The Dark Star Core is located on the top screen and is unreachable. To damage the core, the brothers first have to shatter Dark Fawful's eyes, making him cover them with his two hands and leaving his legs vulnerable. However, it is possible to ignore the glasses and destroy the legs one at a time using Snack Basket. Then the brothers have to destroy all three of his legs (the legs can regrow after a couple of turns). It is possible to destroy them all at once with the Snack Basket technique. After doing so, Dark Fawful will fall and shatter on the ground. Now the Dark Star Core is vulnerable. The brothers have to damage the core as much as possible until he revives Dark Fawful and jumps out of Bowser's body. After he jumps out of Bowser's body, it will jump back into the body of Dark Bowser. Knock out Dark Bowser, make him spit out the Dark Fawful Bug again, then inhale it once more. Keep repeating the process until Dark Fawful is defeated.

Dark Fawful attacks in various ways. Sometimes he picks up the Mario Bros. and shoots rings of stars at them. He will move his arms up and down at various places then shoot the rings. The Mario Bros. have to jump over the rings if they are shot at them. He can also chase after the Mario Bros. and send a bunch of dark stars to attack them. The Mario bros. have to jump over the stars while running from the bug. Before each star hits, it will zoom on the top screen showing either a green or red glow. If the glow is red, it will hit Mario. If it is green, it will hit Luigi. After a while, the Mario Bros. will fall into a white portal and the fight will continue. He can also put both of his arms into the ground and fire balls of energy at Mario and Luigi. The brothers have to destroy the balls by hitting them with their hammers. Sometimes, blue ones will come down. The brothers can hit this one and fire it back at Dark Fawful.

The Dark Star Core uses a different variety of attacks. It can charge up and fire a huge laser at both Mario brothers, knocking them out. To dodge this move, Mario and Luigi have to jump onto Starlow and stay on the top screen. The Dark Star Core will also spin a shadow arrow around it and try hitting the brothers. The arrow will spin in the air right above the Bros. before lowering at the brothers. The Bros. must not jump during this time, then after 5 spinnings in the air as it goes faster and faster, it will lower it at the bros making a visible scrape in the floor, where they have to jump over it. This move is like Cackletta's arm move from Mario & Luigi: Superstar Saga and Elder Princess Shroob's tentacle move from Mario & Luigi: Partners in Time. Once the arrow has spun enough times, it will launch it at one of the bros, who then must deflect it back at the Dark Star Core. This arrow explodes when it hits either Mario, Luigi, or the Dark Star Core. The explosion has a chance to make the bros. dizzy.

After the Dark Star Core is damaged enough, it will be defeated along with Dark Fawful. This will cause Dark Bowser to become unstable. Bowser then lands the final blow on Dark Bowser, ultimately defeating him and saving the kingdom from the Dark Star. After Dark Bowser gets destroyed, Fawful appears before the Mario Bros. one final time. Without a body or his dark power, he doesn't have much time left. He gives one last psychotic smile before self-destructing, which negates the effect of the Vacuum Mushroom and expels Mario, Luigi, Starlow and the Toads from Bowser's body.

Trivia

 * 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.