Dark Fawful is the revamped version of Fawful and the semifinal boss of Mario & Luigi: Bowser's Inside Story. In this form, Fawful has greater powers and an altered appearance, featuring red glasses, a cloak that resembles the one worn by his old mistress Cackletta, and his Vacuum Helmet that seemingly has a life of its own.
Fawful transforms into Dark Fawful after stealing the Dark Star and absorbing some of its power, which increases his own powers. After Bowser defeats Super Peach's Castle, Dark Fawful quickly engages him in battle, which takes place in the Conference Hall, the same area Mario fought Bowser during the first tutorial battle.
Dark Fawful's attacks vary slightly depending on whether or not Dark Fawful has been severely damaged and/or whether his headgear is still present.
The headgear will move Dark Fawful out of the way of Bowser's punch and fire attacks, and will regularly heal Dark Fawful for 300 HP, until it detaches itself from Dark Fawful. After the headgear detaches itself off Fawful, Bowser has to inhale it, allowing Mario and Luigi to fight it. The headgear will remove the bros. Special Attack block, meaning that Mario and Luigi can only attack it by jumping on it. One of the headgear's attacks consists of the headgear shooting out red/green orbs at Mario/Luigi with its extended vacuum. If the body turns red, it will shoot a red orb at Mario, and if the body turns green, it will shoot a green orb at Luigi. Later on, it might also vacuum the Bros.' regular items.
One of Fawful's attacks is to have his headgear grab the chandelier and swing Dark Fawful at Bowser, but this attack can be countered by punching Dark Fawful. After punching him several times, Dark Fawful will swing back behind Bowser in an attempt to ram him, forcing Bowser to duck. An alternative attack involves Dark Fawful pulling out his ray gun and firing eight stars that proceed to orbit around the screen, while one of them will be larger and pink. This star will then turn into the same as the other stars, and Bowser must punch that star to blow all the others up and avoid taking damage. Another attack will involves Dark Fawful flying to the top of the screen and creating four portals - one on Dark Fawful's left and right, one above Bowser, and another one next to Bowser. Dark Fawful will then proceed to toss energy orbs through the portals: if he shoots an orb through the portal on his left, Bowser will have to duck, and if he shoots it through the portal on his right, Bowser has to punch to orb. However, Fawful will sometimes switch the portals next to him around. All of these orbs can lower Bowser's defense if they hit him. The final attack involves Fawful spawning four orbs to surround him. Bowser then has to hit Goombas to Fawful without touching the orbs several times, then the orbs will stop. Hitting one of the dark orbs causes the goomba to fling back at bowser, and failure to do so will cause all of the orbs to attack Bowser, and decreasing Bowser's defense in the process. Later on, Fawful will spawn an extra orb that moves vertically.
However, if Bowser's power is high enough, Dark Fawful can be defeated without his headgear being inhaled. After he is defeated, the Vacuum Helmet will run away. Speaking of the Vacuum Helmet, Bowser's Special Attacks can also target it, in fact completely negating the use of the Vacuum attack.
After he is defeated, Dark Fawful will become unstable, collapse into a bug, and then flee the room. As soon as he enters the next room, he is forcibly inhaled by the incomplete Dark Bowser and becomes the Dark Star Core, which must be battled as a final boss.
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