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I wanted a quasi-original critter to haunt the Carcosa-themed plateau in Kenneth Hite's truly outstanding Qelong setting. The art is from this guy, who apparently just churns out the visual equivalent of solid gold. You should be able to use this guy in 3e or 5e without too much trouble. You'll need to increase attack and damage for the former, certainly. It is designed to be unstoppable, but not necessarily completely lethal to an entire party.
Note that "stun" in my games means that the player loses their move action, not their entire action. So It stuns people telepathically and then eats them, and if it ends up off the plateau for whatever reason, It will start demolishing the campaign setting, hex by hex.