Dreamy Bowser is an enhanced form of Bowser and the final boss in Mario & Luigi: Dream Team. He is created when Bowser inhales the shattered remains of the Dream Stone. While in this form, Bowser is much larger, glows with bright colors reminiscent of those of the late Dream Stone, and has objects resembling the Dream Stone on his head and stomach. "Dreamy Bowser" is also the name for Bowser in the dream world, like Dreamy Mario, who Dreamy Luigi and Mario fought previously. Like all the other final bosses in the series, he is an enhanced or possessed version of Bowser.
When Bowser attempted to use the Dream Stone to erase the Mario Bros. from existence, Starlow and Peach used "the power of good thoughts" to interrupt his attack. At Dreambert's insistence, they then destroyed the Dream Stone to prevent Bowser from using it. However, Bowser inhaled the shards of the Dream Stone, which transformed him into Dreamy Bowser. Now possessing the unlimited power of the Dream Stone, in addition to his own strength, the empowered Koopa King engaged Mario and Luigi in combat.
Upon defeat, Dreamy Bowser explosively expels the remains of the Dream Stone from his body and reverts to his normal form. The Mario Bros. immediately take him out with their Hammers, sending him flying and freeing Peach from her prison. 
One of Dreamy Bowser's more common attacks involves shooting giant, rainbow-colored fireballs, which the Bros. need to jump at the same time to avoid. He clones himself during this attack to shoot double the fireballs; each of the fireballs shot by the clones can only strike a single brother, but the clones fire slightly out of sync, making jumping more complicated for the heroic plumbers.
Dreamy Bowser can also create multi-colored minions from portals to charge at the Bros; after summoning these enemies, Dreamy Bowser will retreat to the background, where he will heal his arms 200 HP and his body 150 HP every turn by eating endless piles of meat until the minions are destroyed. If the Bros. manage to avoid a sufficient number of his minions during an attack, Dreamy Bowser will also slam his fists into the ground to create a shockwave that must be jumped over, and eliminates any of the minions it hits.
A third attack begins with Dreamy Bowser summoning a giant hammer, which he subsequently tries to hit the brothers with; Dreamy Bowser will flip a coin with the Bros. initials on each side to reveal which brother will be attacked first, and thus which brother will first need to pull out his own hammer to stop Dreamy Bowser's hammer.
Dreamy Bowser can also create an Airship with Dreamy Shy Guys (called the Shy Guy Airtub DX) that will chase Luigi as the Bowser head of the Shy Guy Airtub DX fires flames at Luigi. While on said airship, Mario must repeatedly attack the Shy Guy Airtub DX after getting through the Dreamy Shy Guys with his hammer, while Luigi jumps over the fireballs it generates. The Shy Guy Airtub DX will go into the background if it either hits Luigi or survives this attack, and will shoot cannonballs every turn; the left cannon on the ship will shoot cannonballs at Mario, the right one fires at Luigi, and the middle one will simply not hit anyone. These cannonballs will damage the Shy Guy Airtub DX if successfully deflected with a hammer.
Another dangerous attack Dreamy Bowser may perform involves the Koopa becoming giant, forcing the Bros. to flee; Dreamy Bowser will soon start chasing them while spewing 3 massive streams of moving fire that must be jumped over, or the Bros. take massive damage. If the Bros. dodge the fire, Dreamy Bowser will jump and create a shockwave that also must be jumped over. Dreamy Bowser will then repeat the fire stream attack, but his second jump will end in his landing on one of Neo Bowser Castle's many sharp spires, deflating him back to normal size.
Dreamy Bowser's arms are very important in this battle, as he can only perform certain attacks and actions with his arms present. His left arm will periodically be used to grab a brother; the remaining brother cannot use Bros. Attacks as long as a brother is being held, and Dreamy Bowser will throw the brother either that turn or within a few turns if the left arm is still intact, forcing the remaining brother to catch the thrown brother by rapidly pressing their jump button to dash: the thrown brother will take damage every time he bounces off the ground. Dreamy Bowser will simply drop the bro if one bro is KO'd, he decides to turn giant, or he uses an attack that requires both hands (i.e. summoning minions).
His right arm is even more important, as it will prevent Dreamy Bowser's body from taking any damage from aerial attacks while it is still active. Disabling the arm allows the Bros. to land aerial attacks, which will deal critical damage to his head.
Most attempts to attack Dreamy Bowser while he is in the background will result in the brother that initiated the attack being struck with lightning, unless the 'attack' was a Taunt Ball. Unusually for background enemies, using Taunt Balls on Dreamy Bowser or his Shy Guy Airtub DX causes them to perform a specific action, rather than just bringing them into the foreground.
Throwing a Taunt Ball at the Shy Guy Airtub DX will cause it to repeat its opening attack at Dreamy Bowser's orders and then return to the background, defeating the normal purpose of using a Taunt Ball: however, Mario can potentially destroy the Airship during its attack if he's fast enough. Throwing a Taunt Ball at Dreamy Bowser will cause him to jump into the foreground and chase the brothers through the minion crowd if enough of them are left. The Bros. must jump over or on the minions to avoid getting stepped on by Dreamy Bowser, who will also instantly KO minions upon contact. Much like the Shy Guy Airtub DX, Dreamy Bowser will return to the background after he either stomps on a brother or tramples most of his minions.
Dreamy Bowser has the highest stats of any mandatory boss in the Mario & Luigi games, not counting Chakron.
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