- "Soon this kingdom will vanish, along with all who dwell within. And you, too, will sleep eternally in the dark power's embrace!"
- ― Dark Bowser
When the Dark Star absorbs some of Bowser's genetic code, it becomes a form of Dark Bowser, missing the legs due to Fawful having some of the Dark Star power. Later in the story, after Bowser defeats Dark Fawful, Dark Bowser finds Dark Fawful as a bug and inhales him, becoming a whole dark power, due to having the entire Dark Star power. They then go into a spike ball match. If Bowser wins, he may go to the top of the castle in a match with Dark Bowser.
When Dark Bowser is dueled in a battle, Bowser must first attack. Afterwards, Dark Bowser attacks with giant rocks at Bowser and Bowser must punch one at Dark Bowser to prevent Dark Bowser from breathing fire on him and by him gaining the damage instead. Dark Bowser's next attack is locking Bowser in a cage and Bowser must punch the cage lock multiple times to escape the cage so he cannot be hit by Dark Bowser's dark aura attack. Dark Bowser's final attack in his regular form involves him attacking Bowser repeatedly until he faints unless they punch simultaneously to collide their punches. Then, Dark Bowser attempts to use his spike ball attack on Bowser to try to faint Bowser. Bowser must crouch down to avoid it.
When Bowser KOs Dark Bowser, Dark Fawful revives Dark Bowser and Dark Bowser then is able to attack again, but then grows bigger and throws many different types of minions including Goombas, Bullet Bills, Koopas, Thwomps and Bob-ombs in three different patterns during each turn. Dark Bowser then tries to use his spike ball attack on Bowser after Bowser tries to walk along crouching, and punching the dark minions. Bowser must crouch in his shell to avoid Dark Bowser from injuring him. Bowser then must attack Dark Bowser's belly to release Dark Fawful from Dark Bowser's body so Bowser can inhale Dark Fawful for Mario and Luigi to defeat. Dark Bowser also attempts to inhale Dark Fawful.
When Dark Fawful is defeated, Bowser must punch Dark Bowser until Dark Bowser becomes defeated to restore the Mushroom Kingdom to its original state.