You could easily expect any or all of these decks to make an appearance at a locals tournament, or to see them somewhere in the History of the Yugioh metagame
but in that game, he's apparently simultaneously a straight-up alien trapped in the game, a secret unlockable card, or something
(Equinox confirmed that no card would use artwork generated by AI.) Only one unique card will be permitted in each players deck, with commons and rare cards having their own deck construction limitations - although Equinox added that it was experimenting with allowing up to three uniques in a deck
Polteageist is a ghost that lives in a teapot (which can be either authentic or phony), evolving from the previous teacup inhabited as Sinistea
Finally, Duskmourn will also take with it the ally-colored Verge Lands, which have long been a staple of two- and three-color mana bases