John Carpenter's The Thing monster gets isekaid into a typical fantasy world, hijinks ensues with it trying to do its freaky body horror whodonit shenanigans in a world where magic and super powers exist.
Maybe even with the creature trying badly to fit in a fantasy harem scenario, but it just keeps killing the harem girls instead.
While not exactly it, but
Isekai'd Shoggoth is quite close.
I think the Thing would consume most fantasy worlds in a few years. Since its mimicry can be presumed to allow it to use any abilities of the creature it has consumed, it could easily mimic the powerful heroes and wizards of the world, not to mention the superfauna. A flying magic dragon creature that summons storms wherever it moves, but the water is actually just pieces of the parasitic organism. I call that "game over". Additionally, since I presume it is the actual Thing creature, I'd presume it would still retain all of its consumed knowledge, which would allow it to build interstellar level technology as it does in both the film and the original story.
Pretty much an end of the world scenario, there.
if you think about it backroom is just non fantasy version of dungeon huh
less fantasy more of "realism" horror and emptiness
Most fantasy really doesn't do that much with dungeons. Dungeon Meshi is the only one I can think of off the top of my head that really does interesting things with the concept.
Fantasy Backrooms would be an interesting concept. A weird rip in reality to some weird unfinished dimension left behind by the creator god(s) and radiating with the raw energy of creation - still occasionally spawning new horrendous abominations into existence that prey upon anyone that wonders into them and occasionally escape into the world proper. Then you could have every and all monster be a distant descendant of those outside creatures that have adapted into the world or alternatively they could be creatures that have wondered into the BackDungeon, mutated and then dumped randomly back into the world.
I once toyed around with adding Mystery Flesh Pit like creatures as dungeons in my trpg campaign. Living giant creatures made by ancient mad wizards that now roam the world and host entire ecosystems inside them. A neat and quite grotesque take on the dungeon concept, especially when some make them "alive" with the "core" being its heart. Why not make that concept literal? Want to go dungeon delving, better prepare to cut through veins and arteries and a fuckton of gore!