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 some of its power, which increases his own powers. After Bowser defeats Super Peach Castle, Fawful quickly engages him in battle.

Battle
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.

After defeated, Dark Fawful will become unstable, yet soon combined with the incomplete Dark Bowser and become the Dark Star Core, which must be battled as a final Boss.

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.