/Eberron's history involves an empire of Giants on a continent called Xen'drik who became so advanced they could manipulate the planes themselves before they were annihilated by the Dragons for threatening the balance of things.

/casually AUs Thanos as the last survivor of the Giant empire.

Probably hopped planes or whatever. I dunno.

Oh, so he's a planeswalker.

... Hell, MTG settings are apparently being adapted to D&D now so why not

Maybe his thing is annihilating magic or something. Figuring that reaching a certain height with it will just lead to destruction as it did for his kind, the goblins and more recently with the Mourning