Dark Fawful is the penultimate boss in Mario & Luigi: Bowser's Inside Story. He is the darker version of Fawful (hence his name), after Blizzard Midbus is defeated, the Dark Star is complete and Fawful absorbs its power, turning resulting in his transformation into Dark Fawful. Dark Fawful is fought by Bowser.
Upon his defeat, Dark Fawful dies and his ghost (a bug) flees the room.
In battle, Dark Fawful will use his Vacuum Helmet to hang on to the lights and swing on them. Bowser has to punch him three times, them for the fourth time, Dark Fawful will swing around, and Bowser has to duck. In the remake, Dark Fawful instead charges headlong, which must be punched if the boss is coming from in front of his target or ducked if from behind. Dark Fawful can also fire stars from his ray gun. Bowser must use his punch to destroy the big one in order to avoid taking damage. (Doing so causes Fawful to cry.) Dark Fawful can also fire energy into portals and which must be destroyed with a punch or a duck, depending on the portal. During his second phase, Dark Fawful summons energy balls. This can be countered by punching Goombas into him before the boss can throw the dark energy. (as it disrupts his concentration.)
- When Dark Fawful attacks with dark energy balls, his pose looks very similar to his second form from Mario & Luigi: Superstar Saga.
- The song that Dark Fawful sings in Bowser's Inside Story before his battle against Bowser is a parody of the Scooby-Doo theme song.
- During the battle, one of Dark Fawful's attacks is shooting multiple stars that move circularly around the room until counterattacked. This move is similar to the move Princess Shroob uses after passing her half-way mark in her Max HP, but Princess Shroob creates just one star-like attack, while Dark Fawful creates multiple from his ray gun and Dark Fawful's also has a star trail behind.
- It is briefly shown that Dark Fawful's costume underneath his cloak is black and has a picture of the Dark star on it.
- If the Challenge Medal is equipped, the energy ball attack can kill Bowser in one hit especially if the player is underleveled.
- The music that plays when battling Dark Fawful is a remix of Cackletta's battle theme from Mario & Luigi: Superstar Saga & Mario & Luigi: Superstar Saga + Bowser's Minions.