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.
Dark Bowser looks like Bowser. However he has dark blue skin and shell, black horn and spike rings, paler snout, stomach, horn and spike color, red eyes, and a black mohawk.
 Mario & Luigi: Bowser's Inside Story
While the 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 apparently, it had already got every piece of DNA that it needs, and transforms into Dark Bowser, although still in a ghost-like state because he was incomplete. The only parts missing were the legs, which it gathers by inhaling Dark Fawful, thus completing his form, and becoming solid in the process. He flees from Bowser, but when Bowser finds him in the secret tower, they both execute flame breath attacks that clash with each other, initiating the prologue to the final battle. 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 hurricane 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 from the sky. Bowser must duck with his shell to avoid taking damage. However, he must also punch one of the rocks at Dark Bowser, who is charging a massive blast of dark fire. When the rock lands in his mouth, the attack's energy recoils and he inflicts damage on himself from the inside. If Bowser does not, however, he takes damage from the blast.
Another attack involves him punching Bowser repeatedly. Bowser must counter with his own punch, making their fists clash and negate any intended damage. 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 try to land on Bowser. Bowser must bare his shell to protect himself. 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 to launch at Bowser. Bowser must try, by punching repeatedly, and destroy the cage's lock before he gets hit by the energy ball (he should have just about enough time). 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 (assuming Bowser did not make Dark Bowser's HP fall to 0 via counter-attacking) 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 returns to the field, Dark Bowser again curls up into a spike ball and tries to slam into him from above. Bowser must defend with his shell, then do it again as Dark Bowser prepares the next spike-ball slam.
Bowser has to punch him in the stomach so that he spits up Fawful and then inhale him, which Dark Bowser will try to counteract it with his own inhaling technique. 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(s).
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