Dark Star Core

The Dark Star Core is Mario & 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 weak point of the Dark Star. In this form, Dark Fawful has greater powers, along with a slightly altered appearance, such as red glass eyes, changeable appendages, and the Dark Star attached to his head.

History
Following his defeat at Bowser's hands in the conference room, Dark Fawful collapses into a small, insect-like blob of Dark power and flees the room. Bowser follows him into the throne room just in time to see Dark Bowser inhale Dark Fawful's remains, finally granting both access to the full powers of the Dark Star: although he is not seen doing so, it is implied that this is when Dark Fawful became the Dark Star Core. Dark Fawful is not subsequentially seen until the final battle, but it is implied that he assisted Dark Bowser in dueling Bowser on the way to the pinnacle of Peach's castle prior to the final 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 several times previously in the game. If Bowser attacks giant Dark Bowser's stomach, 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, Dark Fawful will become giant sized.

This form of Dark Fawful 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 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 (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 betraying 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.

When 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, Dark Fawful appears before the Mario Bros. one final time as a dark, battered, unstable version of his face. Without either a body or his dark power, he doesn't have much time left. He gives one last psychotic smile before exploding, seemingly negating the effects of the Vacuum Mushroom and expelling 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, 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 backround in the Dark Star Core battle is always be the same no matter which part of Bowser's Body Mario and Luigi are located.