Dark Star Core

The Dark Star Core is Mario and Luigi's final boss of Mario & Luigi: Bowser's Inside Story. It consists of the remains of Dark Fawful after Bowser weakened him, combined with the Dark Star itself. In this form, Dark Fawful has greater powers, along with an altered appearance featuring red glass eyes, changeable appendages, and the Dark Star attached to his head in place of his antenna.

History
Following his defeat at Bowser's hands in the Conference Hall, Dark Fawful collapses into a small, insect-like blob and flees the room; Bowser follows him into the throne room just in time to see Dark Bowser inhale the weakened Dark Fawful, completing the Dark Star's new form. At some point between this and the final battle, the Dark Star merges with Dark Fawful to become the Dark Star Core; while they are not seen until then, it is implied that Dark Fawful assisted Dark Bowser in dueling Bowser on the way to the pinnacle of Peach's castle.

During the final battle with Dark Bowser, Dark Fawful reveals his presence when Dark Bowser is knocked out, reviving him and turning him giant. After Bowser punches the enlarged Dark Bowser, however, he spits out Dark Fawful, who is then inhaled by Bowser and lands inside his body, where he manifests into the Dark Star Core to battle Mario and Luigi.

Eventually, Mario and Luigi triumph over the Dark Star Core, and Bowser then lands the final blow on Dark Bowser, ultimately defeating him and saving the kingdom. Dark Fawful appears before the Mario Bros. one final time as a dark, battered, unstable blob, and gives one last psychotic smile before exploding, intending to take the pair with him; instead, the explosion negates the effects of the Vacuum Mushroom and expelling Mario, Luigi, Starlow and the Toads from Bowser's body.

Battle
During the final battle of the game, Bowser has to make Dark Bowser collapse by dealing 1000 HP of damage. Dark Fawful (now toting the Dark Star on his head) will then fully revive Dark Bowser and cause him to transform into a giant, similar to what had happened to Bowser four times previously in the game. If Bowser attacks giant Dark Bowser's stomach, Dark Fawful will be spat out by the beast and try to flee: Bowser then has to inhale Dark Fawful and let Mario and Luigi battle him. When in Bowser's body, if Bowser fails to inhale Dark Fawful, Dark Bowser will inhale him and Bowser will have to deplete his HP off again. Once inside Bowser's body, Dark Fawful will become giant sized.

This form of Dark Fawful consists of 6 parts: His glasses, three appendages, and the Dark Star Core located on his head. The Dark Star Core is located on the top screen and is initially 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 (Dark Fawful will retract his left and right legs if either of his eyes remain, preventing them from taking damage). Then the brothers have to destroy all three of his legs (the legs can regrow after a couple of turns), a feat most easily done using the Snack Basket technique. Dark Fawful will now fall to the ground and shatter, leaving the Dark Star Core itself vulnerable. The brothers have to damage the core as much as possible before it revives Dark Fawful and escapes Bowser's body, forcing the above process to be repeated.

Dark Fawful's attacks vary depending on whether or not his legs are still present. Sometimes he picks up the Mario Bros. and uses the Dark Star to shoot rings of stars at them. He will move both the Dark Star and his arms up and down at various heights before shooting the rings; depending on where they are, jumping will either have the bros. avoid the attack, accomplish nothing, or actually propel the bros into the stars if the Dark Star is right above them. A second attack involves Dark Fawful transforming into a limbless form that then proceeds to chase the brothers. The Dark Star will try to ram the bros in the meantime, but not before appearing overhead with a colored aura which is either green or red and thus capable of portraying which brother will have to jump. After a while, the Mario Bros. will fall into a white portal and the fight will continue. The last attack Dark Fawful uses sees the loon plant his arms in the ground and send energy orbs down them: the bros. must hammer the energy orbs to avoid a shockwave attack which can knock the victim over, and occasionally a larger orb will be sent down which - if struck - will fly back at Dark Fawful, briefly putting the arm it was travelling on out of commission.

After Dark Fawful himself shatters, the Dark Star will use two new attacks. The first is a tremendous laser similar to that used by the Dark Star's first form which will deal major damage if the bros. fail to grab Starlow before it is used. Its second attack involves creating an enormous dark arrow which spins above the bros. heads before finally descending, forcing the bros. to jump over it: eventually the Dark Star will make the arrow even larger and fire it at a bro: the targeted brother must send the arrow back at the Dark Star with their hammer or else they will take heavy damage and likely be made dizzy. Also, while in this state, the Dark Star is immune to ground-based attacks such as hammers and Snack Basket. In the remake, the beginning of the arrow attack is different. The Dark Star will create black orbs of different sizes and make them spin around it. The brothers will then have to jump over them as they pass by. Eventually, the Dark Star will absorb the remaining orbs to create an arrow with them and fire it at a bro.

When the Dark Star Core is damaged enough, it will be defeated along with Dark Fawful. This will cause Dark Bowser to become unstable.

Trivia

 * When one of Fawful's eyes are damaged, the Dark Star Core closes one of its eyes as well, showing that when Fawful feels pain, the Dark Star does also. This strongly implies that Fawful might be controlling the Dark Star and thus Dark Bowser, an idea further supported by the fact that Dark Fawful tries to flee back into Dark Bowser's body when Bowser inhales him. One thing to be noted also is that Fawful looks visibly upset when the Star Core drags him out of Bowser's body.
 * The Dark Star Core's overworld sprite while having four legs resembles a Dark Satellmite's body structure.
 * The position of the Dark Star in the battle sprite may be a nod to the antenna Fawful gains for the final battle against him in Mario & Luigi: Superstar Saga.
 * The Dark Star Core is similar to Cackletta's spirit form, as they were both villainous Beanish who were turned into undead forms and then fused with Bowser's essence to become twisted versions of him. They are both also final bosses defeated inside Bowser's body by the Mario Bros.

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