- "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
Dark Bowser is the final boss and a darker version of Bowser whom appears in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story DX, where he is the main antagonist. It absorbs Dark Fawful and becomes full with the Dark Star power.
Unlike his normal counterpart, Dark Bowser wants to destroy the Mushroom Kingdom outright.
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.
- HP = 1000 (1500)
- POW = 344 (860)
- DEF = 187 (281)
- Speed = 37 (56)
- EXP = 0
- Coins = 0
- HP = 1 (2)
- POW = 999
- DEF = 999
- Speed = 999
- EXP = 0
- Coins = 0
- Dark Bowser is the third final boss that Bowser has a direct hand in defeating, after Smithy and Super Dimentio from Super Mario RPG: Legend of the Seven Stars and Super Paper Mario, respectively (although using Bowser for either of these battles was optional). The Mario Bros. also defeated Elder Princess Shroob, from Mario & Luigi: Partners in Time, by deflecting Bowser's fireballs up at her (Bowser was unknowingly possessed by the alien princess and she was using him to try and defeat the Mario Bros.).
- Bowser's fight with Dark Bowser is reminiscent of the final battle in Paper Mario, as both take place on a large circular arena above Princess Peach's castle.
- In the North American version, while Dark Bowser is plugged up, he will grow and shake, whereas in the Japanese version, he just grows.
- Like Chakron, when Dark Bowser is defeated, he offers no EXP.
- Dark Bowser, along with the Dark Star Core, is the only boss that can be fought infinitely times on the same save file due to the fact that there is no Save Block after defeating Dark Bowser.
- Dark Bowser is the third final boss that is a different form of Bowser, the first two being Bowletta and Shrowser.
- Surprisingly, those forms of Bowser are all in the Mario & Luigi Series games.
- This is the second time the evil spirit of the main antagonist was fought inside Bowser's body. The first time was in Mario & Luigi Superstar Saga against Cackletta's Soul inside of Bowletta's mouth, as proven by the teeth and the flames.
- One of the glitches in Super Mario Bros. gives Bowser an underground pallet, which has some similarity to Dark Bowser's colors.