The Dark Star is a major antagonist Mario & Luigi: Bowser's Inside Story and its remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. The Dark Star is a sentient artifact of incredible dark power that Fawful eventually seeks out to aid in with the second half of the game focusing on Mario, Luigi, and Bowser's attempts to stop the evil artifact from attaining its full might.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
Long before the events of Mario & Luigi: Bowser's Inside Story, the Dark Star was discovered by Toad archaeologists in the Mushroom Kingdom. Shortly after it was discovered, the Dark Star viciously attacked the kingdom, but was sealed away by the Star Sprites and kept beneath the depths of the world in Toad Town. It was said that if any great evil being broke the seal on the Dark Star and allowed it to attack the kingdom once again, the evil entity would receive enormous power and will rule the world, plunging it into darkness.
Fawful steals the artifact from Toad Town Caves in order to fulfill his plans for global domination. He then extracts Princess Peach from Bowser's body and kidnaps her with the intent to break the seal and awaken the Dark Star; Fawful first uses its power to barricade the front door to Peach's Castle allowing him to use it as a stronghold during the second half of the game. The Bros. eventually used the Miracle Cure after gathering all three Star Cures from the Star Sages to break said barricade and rid all the Toads from the blorbs, allowing them (and later Bowser) to infiltrate the castle in order to stop Fawful from using the malevolent artifact for his plans.
When Bowser finally confronts Fawful and Midbus near the end of the game, the Dark Star finally awakens and tries to escape from the machine that Fawful has been using to drain its power. While Bowser is fighting Blizzard Midbus, Fawful absorbs some of the Dark Star's power and is converted to Dark Fawful; however, Bowser punches Fawful away before he can absorb all of the star's dark powers, and the Dark Star flies into Bowser's body while he is celebrating this exploit. Bowser, suffering from severe heartburn from the star's entrance, inhales some of the frigid air emanating from Blizzard Midbus's frozen body, thus opening the way for Mario, Luigi, and Starlow to enter the previously inaccessible Airway in pursuit of the Dark Star.
At first, all attempts to defeat it failed, with the Dark Star continually slipping out of the intended traps using abilities Bowser would use. Starlow quickly suspects that the Dark Star was somehow copying Bowser's DNA in order to enhance its own powers, an idea confirmed when on several occasions it is seen absorbing some of Bowser's DNA in the Airway (a task which it seems to have disliked). Eventually, the Mario Bros. confront Dark Star at the very back of the Airway. They attempt to destroy it, but it simply escapes from Bowser's body after taking heavy damage. Outside, it transforms into an apparition of Dark Bowser due to some of its power being taken by Fawful earlier and escapes in pursuit of him, freeing Bowser from the room he was trapped in.
Later, when Bowser defeats Dark Fawful, he becomes a small blob of dark power, the Dark Fawful Bug, and flees the room in which he had fought Bowser. The still incomplete Dark Bowser absorbs Fawful and uses the power he took to complete his body.
The Dark Star's final appearance is during the final battle. After Bowser deals critical damage to Dark Bowser, what remains of Fawful heals him fully and causes him to double in size. At this point, Bowser attacks Dark Bowser's belly, causing him to expel Dark Fawful. Bowser absorbs Dark Fawful for the Mario Bros. to handle. Here, he transforms into a giant insectoid mass of dark power, still with his manic grin and now crimson eyes. His only weakness is the Dark Star Core, which sits on an antenna high above his head. As with the final bosses in the previous two games, Mario and Luigi must progressively disable the different parts of Dark Fawful before attacking this weakness. It is not clear whether it still has a distinct intelligence from Fawful's, since when Fawful's eyes close, so do the Dark Star Core's: on the other hand, Fawful appears visibly displeased when the Dark Star Core flees Bowser's body and returns to Dark Bowser. It seems that, because both of them possess dark powers, the Dark Fawful Bug and Dark Bowser now both have limited control over the Dark Star Core's body, despite not being allies.
After the Dark Star Core takes enough damage, it explodes inside Bowser's body, which causes Dark Bowser to become unstable and allows Bowser to finish him off. Dark Bowser then reverts back into the Dark Star (though smaller and slightly purple) before flying above Peach's Castle, exploding and reverting everything back to normal. However, the Dark Fawful Bug managed to survive, and explodes inside Bowser in a last ditch effort to destroy Mario and Luigi. However, all the explosion does is expel the brothers and everyone else from Bowser's body and reversing the chaos Fawful had started in the first place.
The Dark Star reappears in the game's Nintendo 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, with the same role as it had in the original game. It is indirectly referenced in the remake's side-story campaign, Bowser Jr.'s Journey, with Dark Bowser's spreading of darkness occurring around the time Bowser Jr., the Koopalings, and Kamek arrived to stop the Best Fitness Friends at Bowser's Castle, and also witnessing from afar the Dark Star's ultimate destruction at Bowser's hands shortly after the Best Fitness Friends were defeated.
In battle, the Dark Star cannot usually be damaged. To expose its vulnerability, the Mario Bros. must destroy the Dark Satellmites that surround it. With sufficient damage, the Dark Satellmites will use a Kamikaze attack that consists of throwing a bomb at one Mario brother, and throwing their own body at the other. Ordinarily, both will miss, but either brother can attack the projectile launched at him. If the bomb is attacked, it will explode, doing damage to either Mario or Luigi. If the Satellmite is attacked, it will be knocked flying into the Dark Star, which in the original game pierces its' invulnerability and damages it. After being hit in such a way twice, the Star will become vulnerable and immobile for a turn.
The bomb is replaced by an orb of energy in the remake.
One of the Dark Star's attacks consists of it splitting into five (or seven, with sufficient damage) as a thick fog fills the battlefield. Each Dark Star then flies slowly at Mario or Luigi, vanishing into the fog as it draws near. Mario and Luigi must then counter this attack by using their hammers on the hidden fake Dark Stars exactly 2 seconds after they disappear.
In another attack, the Dark Star will shine a bright light at Mario and Luigi, causing their shadows to come to life. The shadows will then move in a circle around the Dark Star, which will attack them with beams of electricity. The Mario Bros. must jump to avoid the electricity that the Dark Star shoots, as any damage done to the shadows will hurt the real Mario Bros. as well.
The Dark Star may also send red and green energy balls into the air. It will then fly through these balls in order, sending them at the correspondingly colored Mario Bro. They may be avoided by jumping.
When the Dark Star is immobilized, it will shoot out a simple laser at Mario or Luigi, depending on who attacked it last.
After receiving sufficient damage, the Dark Star will use a laser attack that fills up the entire touch screen. Starlow will appear just before this attack activates, suggesting that Mario and Luigi grab on to her. When they do so, Starlow will fly up in the air (top screen) therefore avoiding the attack. After the attack is through, the Dark Star will look up and attempt to ram into Starlow. The Bros. must release their hold on Starlow to avoid making contact with the Dark Star. This attack deals massive damage and can inflict the Poison status.
Just before it is defeated, the Dark Star will use a vacuum attack on both Mario and Luigi. This must be countered by pressing and repeatedly or the brothers will take massive damage. After it uses this attack, the Dark Star is defeated. After its defeat, Mario and Luigi can battle the Dark Star X in The Gauntlet.
The Dark Star is initially that of a medium-sized violet orb with a black star displayed on the front. When conscious, the orb grows large spikes that rotate around it, giving it a sun-like appearance. It also has red eyes in the center of the star emblem. When it leeches off of Bowser's DNA, it becomes a shadowy entity that heavily resembles Bowser. After being defeated, it absorbs the Dark Fawful Bug, fully transforming into Dark Bowser. The star itself merged with what remained of Fawful's body, becoming the Dark Star Core. The core itself is a spiky black orb, with a now violet star bearing white eyes and small spikes. After Dark Bowser is defeated and dissolves, the Dark Star appears one last time before it is destroyed. It appears as a blackish-purple star with tired black eyes, surrounded by a weak, purple aura. However in the remake, it reverts into its initial state with the aforementioned purple aura before it is destroyed.
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