Mario and Luigi: Bowser's Inside Story -- Part 21: If only it were under Peach's Control

The Dark Star transformed into Dark Bowser and is looking for Fawful, who has drained much of its power. Go south and you'll see Dark Bowser break open the castle's front door. Go north first and grab the items in the garden and conference room, as well as other rooms. When you're ready, hit the Save Block and exit the castle through the front door.

"Dark Star! Dark Star! Fawful is wanting you!" Fawful appears, wearing his headgear and a new outfit with the Dark Star on it. Bowser calls him a pain. "It is you who are the pain to me! I am needing more power! Your meddling has made Fawful have a fury large as the world! Bowser! It is time for meeting and greeting with your doom! For now it is the end of the final act of the finale’s closing encore! I am having this castle! And now I am thinking... It needs Fawfulization! Time for the remodeling! SUPER PEACH'S CASTLE OF FURY!" The ground starts shaking. The castle slowly transforms into a giant robot. It stomps Bowser, leaving him unconscious.

Go to Rump Command and save your globin, then ford the river of adrenaline. Once Bowser is revived, he once again becomes giant. The Toads in Toad Town freak out. The castle fires off dark energy into the sky. It lands behind Bowser, creating a black hole. After Bowser attacks, it will trying drawing him in. Slide the stylus repeatedly to the right to run away. The castle will try to punch Bowser, so slide the stylus to defend. If you breathe fire, the castle will throw out its flammable ballast, which is mushrooms. The castle will also send out Mechawfuls. Shoot them down with fireballs. Breathe fire when it sends out an army of Mechawfuls. It will launch an unavoidable attack, which is a laser that will knock Bowser into the black hole. Slide the stylus to get out or you will slowly take damage. When Bowser flies out, he'll fly into the castle, doing extra damage. It will try to create another black hole, but Bowser knocks it behind the castle. The castle will put up a force field, so it won't take damage, but you can still move it. Bowser will roll into a ball. Move the stylus in a clockwise circle to push against the castle, which gets it stuck in the black hole. The force field will be destroyed. Keep pushing it into the black hole when the force field is up. Punch the missiles it launches. When it presents spikes, slide up with the stylus to make Bowser jump. When the castle has half of its health left, the black holes move much closer, meaning any hit will send either of them into a black hole. Later, its invisibility field will activate. This means it will turn invisible when it fires out of a black hole. Time your attack well. When its health is very low, both it and Bowser will be sucked into the black holes. Launch Bowser out of the black hole repeatedly at the castle, timing it so that the castle's spikes are not bared. Keep up the attack until the castle is defeated.

Bowser says that was a fight he didn't expect. Dark Bowser appears, then disappears after Bowser tells him to give back Peach. He explains the situation to Chippy, who tells the bros. She says the only way to win is to team up with Bowser, even if he doesn't know it. She says to prepare and finish anything they want to get done. Send the bros into the castle via the front door, then go to the right. Go north into rooms where you can get items. There's running water on the east side, which can be used to extinguish the fire in the fireplace. You can items in the area north of there. You can drill under the stairs to move around. Now is the time to level up. Bowser can train in Peach's Castle and the bros should train in the Airway, when Bowser is near Midbus.