Dark Fawful is the revamped form Fawful takes after absorbing most of the Dark Star's power, who is the penultimate boss of Mario & Luigi: Bowser's Inside Story and its remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. 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.
While his battle in the original game uses the standard boss battle theme, in the remake Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, Dark Fawful has his own battle theme, which is a slowed down arrangement of Cackletta's battle theme from Mario & Luigi: Superstar Saga and Mario & Luigi: Superstar Saga + Bowser's Minions.
After stealing the Dark Star from Toad Town Caves and taking over Peach's Castle, Fawful and Midbus oversee the evil artifact's awakening, using Peach's power to break the seal. However, Bowser manages to enter the castle and reach the room where the two are at. After defeating the newly-powered Blizzard Midbus, the Dark Star awakens, and Fawful uses his headgear to absorb its power, transforming into Dark Fawful. Before he could absorb all its power, Bowser punches him away.
Later on, after following the incomplete Dark Bowser into Peach's Castle Garden, Bowser meets up with Dark Fawful. He sends out the Fawfulized Super Peach's Castle of Fury after Bowser, whom the latter defeats after Mario and Luigi help him to become giant to battle the castle, returning it to normal. The next time Bowser encounters Dark Fawful is when he is searching for the Dark Star so he can absorb the rest of its power. He corners him in the Conference Hall; the place where he first fought Mario back in the beginning of the game. Having enough of Bowser's meddling, Dark Fawful fights Bowser, but is defeated.
After the battle, Dark Fawful sustains too much damage and turns into a small insectoid-like blob creature. He flees the hall, though he is inhaled by Dark Bowser shortly afterwards, completing him. It is assumed that he merged with the Dark Star within Dark Bowser's body to turn into the Dark Star Core. During the final battle between the two Bowsers, the Dark Star Core helps Dark Bowser out by fully recovering and turning him giant once Bowser reduces his health to zero, though it is spat out of his body once Bowser punches his stomach. Bowser inhales the Dark Star Core, forcing Mario and Luigi to battle it, destroying its legs in order to attack its weak point; the Dark Star itself.
Once the Dark Star is defeated, it explodes and leaves Dark Bowser unstable, giving Bowser the opportunity to defeat him, returning the Mushroom Kingdom back to normal. Meanwhile, Fawful is shown to have survive, albeit very weakened. As a last ditch effort to destroy Mario and Luigi, Fawful feigns accepting defeat and explodes inside Bowser's body with a wide grin on his face. However, the explosion reverses the effects of the Vacuum Shroom and actually frees both brothers, Starlow, Toadsworth, Toadbert and many other Toads from his body.
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 can choose to inhale it, allowing Mario and Luigi to fight it.
One of Fawful's attacks is to have his headgear grab the chandelier and swing at Bowser, which can be countered by punching Dark Fawful multiple times. After punching him several times, Dark Fawful will swing back behind Bowser in an attempt to ram him, forcing Bowser to duck.
Dark Fawful can use his ray gun to create an array of stars in a ring formation, with one star being a red one instead of purple. This star will then turn into the same as the other stars, and lower down to the point Bowser has to duck. Bowser must punch the correct star to blow all the others up and avoid taking damage.
Another attack involves Dark Fawful flying to the top of the screen and creating four portals - one on his left and right, one above Bowser, and another one next to Bowser. Dark Fawful will then proceed to toss energy balls through the portals: if he shoots a ball 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 the ball. Later in the battle, Fawful may switch the portals next to him around.
The final attack involves Fawful spawning four orbs to surround him. Bowser has to time his punches and punch Goombas through the gaps in the orbs to stop the attack. Hitting one of the dark orbs causes the Goomba to fling back at Bowser, and failure to repel the attack will cause all of the orbs to attack Bowser. Later on, Fawful will spawn an extra orb that moves vertically. The star, energy ball and orb attacks may cause Bowser the DEF down status effect.
Bowser can use his special attacks on the Vaccum Helmet as well. 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.
In the remake, Dark Fawful's chandelier attack is replaced with a speedier attack; he will swoop at Bowser, who must punch him out of the way. Dark Fawful will zoom in from the back, to which Bowser must duck, and finally zoom in from the front again, to which Bowser must punch once more. Dark Fawful's Vaccum Helmet also heals him for 500 HP instead of 300 each turn.
Names in other languages