Nothing but darkness is where we begin.
Antasma was unconscious and floating in the dark, With one reoccuring nightmare haunting him, "Defeated, Me, By the hands of those.... PLUMBERS!!!!!!!"
"Mario, Luigi, And you too, Dreambert! I'm gonna get out of vis place and send you to hell where you belong!!!!" He said in his trademark warped-voice Transylvanian accent.
"You cannot go back." Said a deep voice that even gave Antasma chills.
"Who-Who's vere?" He said with signs of worry on his face.
"Mario and Luigi have clearly sealed you off from their world, But you have another chance in mine." Said the voice once more.
"Vere are you going with vis?" He asked back.
"I reside in Equestria, The land of talking mythical horses, And you would make a perfect ally." The voice revealed himself as a black unicorn with a red tinted horn.
"OH NO! I don't vork with allies, Not after last time." Antasma said bringing up the memory of Bowser betraying him all those years ago.
"Very well then, But you must know one thing first." The unicorn said to him. "I am King Sombra, The Lord Of All Fears, And i know what you are, Antasma The Bat King."
"How do you know me?" He said back in shock.
"I watch more than just Equestria, I watch other worlds as well, I know you were defeated by a Pi'llo and 2 plumbers." King Sombra said to him.
"Mario, Luigi, And Dreambert." He said to Sombra with a hint of fury running down his spine and out his tail.
"And if you try to rival me, So help me Celestia, I will make sure you don't get a 3rd chance!" Sombra fumed at him.
"And just vat do you think you can do?" He said as if to challenge the unicorn's power.
"I create perfect copies of fear and use it against my opponents" Sombra said.
"Big voop, I manifest on nightmares, Which in turn make me all powerful!" Antasma said back as a snappy retort.
"Beaten by a heart!"
"Beaten by plumbers!"
The argument went on for quite some time, until Sombra gave up and transported Antasma to the Everfree Forest, Where his new conquest would begin.
"Remember, You'll be seeing nothing but ponies in this world, That and a few other animals that aren't related to equines in any physical way." Sombra warned him.
But all Antasma did was grin, He knew it was a relief from fighting humanoid creatures and humans, "Vere's a princess ruling vis land is vere not?"
"Yes, Princess Celestia and her sister, Luna, They both control all of Equestria, And like the saying says: Cut off the head, And the body will die!" Sombra informed him.
"Excellent, New world, Check, New powers, Check, New princess to capture and or kill, Check, I'd say i'm ready to rule all of vis vorld with an iron fist!" Antasma finished laughing.
I have some ideas for this story.
After his betrayal by bowser antasma is now dangerously genre savvy, thus he is more cunning, better at planning, more pragmatic at combat and is much more skilled in a bit of everything else,
At night antasma goes into ponyvil and feed on the nightmares of ponies (he absorb power trough the nightmare). If no nightmare are currently happening antasma goes into a good dream and turn it into hell, every victim claim they see a bat like shadow in their nightmares, eventually antasma torments celestial by turning her perfect dream into total hell, this cause celestial to be paranoid and twilight goes apeshit,
Antasma use fear as of his primary advance and sometime will fight dirty, "I do not fight to make my enemies happy about losing" like always he still can open portals to the dream world.
His hideout is in an enormous cave in wich he rules as the king of the bat ponies living in it, he probably created a castle in it with his power, (maybe he has an adoptive daughter? A filly, the only being antasma is kind and shows love to,)
Antasma often summons a ghostly song out of nowhere (his battle theme) when fighting to cause discomfort of fear in his opponent,