Dark Bowser is Bowser's final boss in Mario & Luigi: Bowser's Inside Story. He has all the powers Bowser has, along with many dark variations as well. He is far more evil than the original Bowser; while Bowser simply desires to take over the Mushroom Kingdom, Dark Bowser wants to destroy it outright.
 Mario & Luigi: Bowser's Inside Story
While the evil Dark Star was gathering DNA from Bowser's body, Mario and Luigi tried to stop it before it succeeded. The brothers manage to defeat it, but it seems that it had already got every piece of DNA that it needs. The only parts missing were the legs, which it gathers by inhaling Dark Fawful, thus creating a dark version of Bowser. He flees from Bowser, but when Bowser finds him in the secret tower, they both breathe fire at the same time, starting a duel. Bowser overwhelms him, then they both curl up into a spike ball and the fight commences. Bowser again overwhelms his dark form, who then unleashes a dark storm over the Mushroom Kingdom, trapping Broque Monsieur, the Blue Paratroopas and several Toads in bubbles. Bowser is now the only one who can defeat him.
One of Dark Bowser's attacks is throwing a boulder into the air and blasting it apart, causing rocks to fall out of the sky. Bowser must use his shell to avoid taking damage. However, he must also punch one of the rocks at Dark Bowser, who is charging an attack. When the rock lands in his mouth, it plugs him up and causes his attack to damage himself from the inside. If Bowser does not, he is blasted by dark fire.
Another attack involves him punching Bowser repeatedly. Bowser must counter with his own punch, making their fists clash. The two giant blows meet and throw the titans back, creating a shock wave. Then Dark Bowser will jump into the air as a spiked ball and tries to land on Bowser. Bowser must retract into his shell to avoid taking damage. He can then counterattack by quickly throwing a punch at Dark Bowser. Another attack traps Bowser in a cage and raises it into the sky. He will then charge up an energy ball. Bowser must try and destroy the cage's lock before he gets hit by the energy ball. He can also swing the cage off its hinges with shell defense to damage Dark Bowser.
After taking enough damage, Dark Bowser will fall to the ground; he is then healed by the ingested Dark Fawful and will grow larger. He will always follow this up by tossing a mixed ball of "Dark Minions" at Bowser, including Goombas, Koopa Troopa, Thwomps, Bob-ombs, and Bullet Bills. The ball pushes Bowser back several feet, after which he must then walk back through the "Dark Minions" while dodging or countering their attacks. When he catches up, Dark Bowser again curls up into a spike ball and tries to body slam him. Bowser must curl up into his shell, then do it again as Dark Bowser prepares the next body slam.
Bowser has to punch him in the stomach so that he spits up Fawful and then inhales him, which Dark Bowser will try to do as well. Successfully inhaling Fawful pulls him into Bowser, where the Mario Brothers must fight Dark Fawful. Only upon Dark Fawful's defeat can Bowser give his shadowy doppelganger the final blow.
Once defeated, Dark Bowser dissolves, leaving only the Dark Star, which rises into the air and explodes, restoring the Mushroom Kingdom to normal.
While Dark Bowser is incomplete, he speaks extremely slowly. However, as he becomes complete, he starts to speak fully. In the Japanese version, his speech is halted and written completely in katakana while he is incomplete. When Dark Bowser inhales Dark Fawful and is completed, he begins to speak in more natural Japanese, with kanji and no pauses.
 Names in Other Languages