Dark Bowser is the form the Dark Star takes after absorbing enough of Bowser's DNA, who acts as the final boss in Mario & Luigi: Bowser's Inside Story and its Nintendo 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey (more accurately, Bowser's final boss). His voice is a slowed-down version of Bowser's, and he has all the powers the original has, along with many dark variations as well.
When the Dark Star was gathering DNA from the Airway inside Bowser's body, Mario and Luigi tried to stop it before it succeeded. The brothers fought hard to defeat it, but apparently, it had already absorbed every piece of DNA that it needed, and it transformed into Dark Bowser, although in a ghost-like state because he was incomplete. The only parts missing were the legs, which it obtained by inhaling Dark Fawful (he inhaled an unconscious Princess Peach earlier, but nothing happened), thus completing his form, and becoming solid in the process. He flees from Bowser, but when Bowser finds him alongside a still unconscious Peach (who is now inexplicably outside of Dark Bowser) in the secret tower of Peach's Castle, they both execute flame breath attacks that clash with each other, initiating the prologue to the final battle. Bowser overwhelms Dark Bowser, then they both curl up into a spike ball and the fight commences. Bowser again overwhelms his dark form, who gets away. Later, Dark Bowser unleashes a dark hurricane over the Mushroom Kingdom, trapping Broque Monsieur, the Koopa Paratroopa Trio and several Toads and Goombas in bubbles. Bowser is now the only one who can defeat his doppelganger.
Once Mario and Luigi defeat the Dark Star Core inside Bowser, Dark Bowser becomes unstable. This allows Bowser to finish him off with a "one KO punch". Dark Bowser then turns into a star that flies above Peach's Castle and explodes, reverting the kingdom back to normal.
Dark Bowser has all of the original Bowser's physique and build, but is detailed very differently. All of his scales and shell are colored dark blue, and his snout, stomach, horns and spikes are noticeably paler than Bowser's. His eyes are identical to the Dark Star's own, but seems to entirely lack Bowser's eyebrows. The rings around his spikes and horns are colored black, and the most noticeable difference from Bowser and Dark Bowser is that Dark Bowser has a black mohawk. His sprites also has dark orbs float across his body.
In his incomplete state, he retains his details, but appears as a ghostly figure with a purple coloration (without his eyes), whilst pulsating a brighter purple. His animations are also distinctive in that his idle sprite has misplaced pixels to give an effect resembling video noise, and his talking animations have a noticeable jitter to them. He also entirely lacks legs and a tail until he is fully complete.
While Dark Bowser is incomplete, his speech is halted and can only speak one sentence at a time. However, upon his completion, he starts to speak fully and does not use contractions.
In the Japanese version, his speech is halted and written entirely 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.
Aside from text dialogue, Dark Bowser's roars and grunts are provided by Kenny James, who also voices the original Bowser.
Powers & abilities
While incomplete, the only powers Dark Bowser is shown to possess are floating, punching and inhaling. Once he inhales Dark Fawful after he is reduced to a bug-like form and turns complete, Dark Bowser showcases even more powers that are similar to the original Bowser's; exhaling/shooting fire out of his mouth, curling up into a spike ball, etc. Dark Bowser also has the ability of spreading darkness throughout large areas of land, seen before the final boss fight with him, which also traps any residents inside dark bubbles.
During the final battle, Dark Bowser has the ability to create boulders, cages and energy balls as the means to attack Bowser. With the help of Dark Fawful, who is inside him, Dark Bowser can recover any and all damages given to him, seen once Bowser reduces his HP to zero, and even grow larger (with his belly being his only weak spot, which makes him cough up Dark Fawful and revert him back to normal size). Also, Dark Bowser makes use of dark variations of Bowser's own minions, sending them after Bowser when he turns giant.
However, if the Dark Star Core takes too much damage, Dark Bowser will become weakened and unstable, which is what ultimately allowed Bowser to finish him off during their fight.
One of Dark Bowser's attacks is throwing a boulder into the air and blasting it apart with fire, 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 in front of him 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 do this, 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 negating any intended damage. Afterward, Dark Bowser will jump into the air as a spiked ball and try to land on Bowser. Bowser must duck in his shell to protect himself. He can then counterattack by quickly throwing a punch at Dark Bowser after Bowser ducks. Another attack traps Bowser in a cage and raises it into the sky. Dark Bowser will then charge up an energy ball to launch at Bowser. Bowser must try, by punching repeatedly, to destroy the cage's lock before he gets hit by the energy ball (he should have just enough time to do this). He can also swing the cage off its hinges by ducking into his shell to damage Dark Bowser, however, this method takes longer and requires precise timing while swinging.
After taking enough damage, Dark Bowser will fall to the ground; he is then fully 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 versions of Bowser's minions (Goombas, Koopa Troopas, Thwomps, Bob-ombs, and Bullet Bills) at him. The ball pushes Bowser back several meters, after which he must then walk back through the minions while dodging or countering their attacks (punching the Goombas, Koopa Troopas, Bob-ombs and low-altitude Bullet Bills, and ducking under the high-altitude Bullet Bills and Thwomps). When near to the battlefield, 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 can avoid the second attack just by walking.
When it is his turn, Bowser has to attack his stomach so that he spits up the Dark Star Core and causing Dark Bowser to shrink back to normal size. Bowser must then inhale Dark Fawful, which Dark Bowser will try to counteract it with his own inhaling technique. Fail this part and Bowser will have to do it all over again. Successfully inhaling Dark Fawful pulls him into Bowser's belly, where the Mario Brothers must fight Dark Fawful. If Mario and Luigi fail to defeat Dark Fawful and Dark Star Core within several turns, he will escape and return back into Dark Bowser's body, forcing the process to be repeated. Only upon Dark Fawful's defeat can Bowser give his shadowy doppelganger the final blows. Once defeated, Dark Bowser dissolves, leaving only the Dark Star, which rises into the air and explodes, restoring the Mushroom Kingdom to normal.
Names in other languages