Dark Fawful

Dark Fawful is a stronger form of Fawful, and the final boss of Mario & Luigi: Bowser's Inside Story, along with Dark Bowser. 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 Headgear that seemingly has a life of its own.

Mario & Luigi: Bowser's Inside Story
Fawful transforms into Dark Fawful after stealing the Dark Star and absorbing half its power, which increases his powers. He engages Bowser in battle after this.

First Form
While he is fighting Bowser, he has many abilities. While he has his Headgear, he can attach himself to the chandelier on the ceiling and swing himself at Bowser. While he swings at Bowser clockwise, he will need to punch him away. If he swings counterclockwise, Bowser will need to duck. His headgear can also heal him for 300 HP and cannot be attacked by Bowser normally. Another one of Dark Fawful's attacks is using a ray gun to fire a beam at Bowser. The player will need to watch it carefully in order to decide if they need to duck or punch it. If one of the stars is bigger than the others, then Bowser will need to punch the beam. If not, then they need to duck. Dark Fawful can also open up four portals and toss a ball of dark energy into them. If he tosses it into the left one, Bowser will need to block it with his shell. If he tosses it through the right portal, he will need to punch it. If Fawful switches the portals adjacent to him around, then Bowser will need to do the opposite. If Dark Fawful surrounds himself with multiple balls of dark energy, Bowser will need to quickly punch Goombas at him, or he will take massive damage from the attack. However, he can do significant damage to Dark Fawful while doing this. After Bowser inhales his headgear, it will attack the Mario brothers. First it will steal either their Item blocks or Special blocks (preventing them from using items or special attacks) and then trap one of the bros in a strange device, then swing the device at the other bro. The freed brother will need to jump on the device to free the trapped brother or he will steadily take damage. It can also pump energy balls to shoot at the Mario Bros. It changes color while it is doing this to determine what brothers it will attack in succession. Fortunately, if they do enough damage to the Headgear, it will give them their item commands back.

Second Form
After being defeated, Dark Fawful loses his body and transforms into his second form to escape from Bowser, but gets inhaled by the leg-less, incomplete Dark Bowser, which allowes him to complete his body.

In the final fight of the game, Bowser has to make Dark Bowser collapse by making him lose 1000 HP. Dark Fawful will then recover him and make Dark Bowser giant. Bowser then has to attack the giant Dark Bowser's stomach to make him spit out the Dark Fawful. 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.

Dark Fawful's second form 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 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. Then the brothers have to destroy all three of his legs (the legs can regrow after a couple of turns). It is possible to destroy them all at once with the Snack Basket technique. After doing so, Dark Fawful will fall and shatter on the ground. Now the Dark Star Core is vulnerable. The brothers have to damage the core as much as possible until he revives Dark Fawful and jumps out of Bowser's body. After he jumps out of Bowser's body, it will jump back into the body of Dark Bowser. Knock out Dark Bowser, make him spit out the Dark Fawful Bug again, then inhale it once more. Keep repeating the progress until Dark Fawful is defeated.

Dark Fawful attacks in various ways. Sometimes he picks up the Mario bros. and shoots rings of stars at them. He will move his arms up and down at various places then shoot the rings. The Mario bros. have to jump over the rings if they are shot at them. He can also chase after the Mario bros. and send a bunch of dark stars to attack them. The Mario bros. have to jump over the stars while running from the bug. Before each star hits, it will zoom on the top screen showing either a green or red glow. If the glow is red, it will hit Mario. If it is green, it will hit Luigi. After a while, the Mario bros. will fall into a white portal and the fight will continue. He can also put both of his arms into the ground and fire balls of energy at Mario and Luigi. The brothers have to destroy the balls by hitting them with their hammers. Sometimes, blue ones will come down. The brothers can hit this one and fire it back at Dark Fawful.

The Dark Star Core uses a different variety of attacks. It can charge up and fire a huge laser at both Mario brothers, knocking them out. To dodge this move, Mario and Luigi have to jump onto Starlow and stay on the top screen. It can also fire a shadow arrow at either Mario or Luigi. The brother has to hit the arrow with his hammer. The Dark Star Core will also spin a shadow arrow around it and try hitting the brothers. The Mario bros. have to jump over the arrow when it gets near them. This move is like Cackletta's arm move from Mario & Luigi: Superstar Saga and Elder Princess Shroob's tentacle move from Mario & Luigi: Partners in Time.

After 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, Fawful appears before the Mario bros. one final time. Without a body or his dark power, he doesn't have much time left. He gives one last psychotic smile before self-destructing, which negates the effect of the Vacuum Mushroom and expels Mario, Luigi, Starlow and the Toads from Bowser's body.

Trivia

 * When Dark Fawful attacks with dark energy balls, his pose looks very similar to his second form from Mario & Luigi: Superstar Saga.
 * When Bowser is fighting Dark Fawful in his first form, he is fighting in the same spot he fought Mario in the beginning of the game.
 * When one of Fawful's eyes are damaged, the Dark Star Core closes one of its eyes as well.
 * Dark Fawful when fighting with the Dark Star's Core is like an equivalent to Super Dimentio from Super Paper Mario, except a bit trickier to defeat. Both die in a similar way as well, as in their final moments they are reduced to just a head.
 * It is actually possible to defeat Dark Fawful without destroying his Vacuum Helmet, if Bowser uses special attacks.