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 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 hair/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 with the Dark Star attached to 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; taking damage from an orb will cause it to disappear. 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

 * The position Fawful finds himself in in the final battle is a direct parallel to situation he forced on Mario & Luigi, both being inside a vessel and giving them aid from within, which is made more obvious by the fact that both vessels are Bowsers.
 * It is not entirely clear whether Fawful or the Dark Star is in control of their combined form as Fawful still acts the same as he always has and seemingly is able to control the Core to use its powers (seen by him charging it to heal Dark Bowser, grow big or shroud the arena in darkness). At the same time the Core clearly isn't just a tool for Fawful, also acting more as a separate entity supporting him and still fighting even when he is downed. It is also able to forcibly reabsorb Fawful and leave Bowser, much to Fawful's visible dismay, showing that it has at least some control over him.
 * 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.
 * The Dark Star Core is similar to Cackletta's spirit form, as they were both villainous Beanish who were turned into undead forms after losing a battle, then fused with Bowser's essence to become twisted versions of him and were fought and killed inside him.
 * Despite being commonly referred to as the Dark Star Core, this form as a whole doesn't actually have a name as the Dark Star Core is just the name of the form's core and Fawful in his entirety cannot be targeted, only his legs and glasses.
 * If Dark Bowser is KO'd while Dark Fawful is outside of it, Fawful will run back onscreen and revive it before running offscreen again.
 * The Dark Star Core's overworld sprite while having four legs resembles a Dark Satellmite's body structure.
 * The position of the Dark Star attached to Fawful's head by an antenna of black electricity may be a nod to the bulbous antenna Fawful gains for the final battle against him in Mario & Luigi: Superstar Saga, supported by Dark Mechawfuls having a star antenna in place of the normal Mechawful's bulbous antenna.
 * The Dark Mechawfuls fought near the end of the game also likely foreshadow Fawful's core form as they have star antennas that can shoot stars and lasers and when beaten lose their body and become little more than a head with two claw-like arms that can only be fought by the Bros. after being inhaled by Bowser.
 * In the remake of Bowser's Inside Story a few things are changed about the battle with the Dark Star Core: Fawful will now grab the ceiling and retract all of his legs if the player tries to use a multi-target attack while his eyes are still intact, the swinging arrow attack is replaced with dark orbs although the dark missile part still remains and he only uses the chase attack after the second time he's inhaled.
 * Also in the remake, the battleground for the fight between the Mario Bros. and the Dark Star Core looks drastically different. Compared to the original which was full of alternating rainbow colors and a pulsating cell-like structure in the background, the remake's has a darker palette composed mostly of pinks, purples and blues with organic Fawful face holes in the flesh and a Bowser shell shaped structure in the background that beats like a heart.
 * Other more minor changes introduced in the remake include Fawful laughing and pointing at the Bros. before turning into his giant form, throwing the Bros. in the air and catching them before doing his hands attack instead of picking them up, he now has arms that he uses to drag himself forward in the chase attack instead of just being a head, the Core elevating itself to the top screen before creating the dark missile instead of staying on the bottom screen with shattered Fawful and the music no stopping after the Core's defeat.

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