The Nightmare Whale. An insane, screaming, albino, telekinetic whale. Yeah Moby Dick had psychic powers, you surprised? Surface beings rarely meet these creatures from the under-ocean. Sometimes they go fucking crazy and get trapped in a nightmare they can't wake up from. Their telekinesis means they lift out of the ocean and, vertical in the air, heave screaming through the underdark, wailing crazed songs in ultrasonic.
You could try killing them, or waking them up. But who understands the dreams of whales? Surrounded by the force-ghosts of squid the whale is imagining tormenting it, and which are actually attacking everything around. Not a city destroyer, but maybe a killer of towns.