https://youtu.be/WMdkPoGFJ34

The camera pans over the outside of Neo Bowser Castle before cutting to a view of the door to Antasma's Nightmare world as Bowser punches it open, stepping inside.  The Koopa King notices a figure sitting on the throne at the end of the chamber. Undaunted, he steps forward, channeling flame in his mouth while taunting the unknown entity as he speaks.

Bowser: Your castle is lost, Brat King! No being can challenge my power.

The camera cuts to the being on the throne, shrouded in shadow, who is revealed to be Antasma himself, holding a glass of 'Grape Fanta' soda. He simply laughs at Bowser's bravado.

Antasma: But vhat is a TRAITOR?!  Fucking USELESS!  SKREEHEEHEEHEEHEEHEE!

He throws his tar-colored drink, as King Koopa charges at him with a King's Choke to Antasma's shock. As he strikes, an explosion decimates the Castle Lord's throne. Bowser gets up, before noticing that his enemy is nowhere to be seen. Antasma proceeds to teleport behind the turtle as Bowser turns back to speak once more.

Bowser: I'm not the traitor here...

Bowser hovers towards the vampire, preparing to punch him, but Antasma easily teleports out of the way before laughing and reappearing behind the Koopa, slashing his face with a vicious swipe of his claws and drawing blood. Bowser strikes with a Koopa Punch straight to Antasma's chest. He smirks at the successful attack, before realizing that the Lord of the Castle has turned into a swarm of bats to nullify the attack. Antasma's head forms out of the horde of bats to laugh, as the swarm ascends to the top of the room and reforms back into the vampire's full body. Antasma smugly smiles as he charges his Shadow Blast, firing it at Bowser and blowing his opponent away. Bowser quickly recovers, though he doesn't notice a group of Flaming Antasmatons coming from behind.

Bowser: Your tricks mean nothing.

As he says this, a Flaming Antasmaton comes from behind and hits him. Bowser then proceeds to swat one of the Flaming Antasmatons out of the air and smash it into the ground, although the rest overwhelm him. At this point, Antasma teleports in and lifts Bowser by the neck. In return, using his Arm Center, Bowser blows Antasma back, before driving him into the ground with King's Choke, then hits him with a Koopa Punch to send him flying. Bowser then follows up with a barrage of fireballs, but Antasma recovers, and sends said fireballs flying back at Bowser with the Force. In return, Bowser takes out his Goomba Storm, and rushes Antasma, deflecting two of the energy blasts with two of his Goombas, before slashing Antasma's chest. Bowser smirks for a moment before Antasma begins laughing, and at this point, Antasma transforms into his nightmare form.

Antasma: Despair.  SKREEP!

Bowser stares in horror for a moment, before attempting to charge the nightmare behemoth and promptly getting tossed around like a ragdoll, including losing the ability to use Goomba Storm. Antasma then attacks Bowser with his Antasmunchie Shock Wave, causing him to scream out in pain. Then, however, Bowser's eyes begin to glow white, and a burst of energy erupts, dispelling the hellfire. From within, due to the Rump Command, Giant Bowser emerges.

Giant Bowser: Behold true power!

Bowser then rushes down Antasma, and swipes at him with his claws, briefly overwhelming the transformed vampire. After being briefly overwhelmed, Antasma begins charging up a devastating magical attack: the Nightmare Loop.

Antasma: I am no simple Bat King; I am a nightmare god!  SKREEYEE!

As he says this, Antasma unleashes the Nightmare Loop, which threatens to overwhelm Bowser. In response, Bowser attempts to flee back with a good portal, but there are none, and Bowser is overwhelmed. Neo Bowser Castle explodes from the sheer power of the Nightmare Loop, and Bowser is shown to have returned to his normal size, laying dazed in the rubble. Antasma's nightmare form approaches him and grabs him by the head.

Antasma: You're right... You're not a traitor, you're a mere turtle. Time to make soup outta you.

Bowser attempts to call upon his Rump Command again, but this time, nothing happens, and Antasma takes the moment to ignite his lightsaber and slice the pathetic turtle in two vertically, ending his life. Antasma then holds his foe's HEAD over a pike, and staked it.  Meanwhile, the Goomba Storm members lay DEAD in the ground in front of Antasma.

Antasma: That's vhat you get for betraying me.  Now I rule over your subjects!  SKREEHEEHEEHEEHEEHEEHEEHEEHEEHEEHEE!

The battle pans out as Shy Guys and Koopas bow to Antasma, and in return, Antasma gives them the pay Bowser promised them.