Vaporighu

Picture a swollen, vaguely human shape with blotchy pink-to-purple skin slick with corrosive ooze. Arteries squirm visibly under that skin, and clumps of matted, death-reeking fur cling to its forearms, shins and chest. It breathes with a loud, wet wheeze like a leaking bellows, and its stench clings to a place for days. It speaks no tongue known to humans and would have little to say anyway.

Simply seeing one is terrifying. Anything of 3 HD or fewer saves vs. spell or cowers, paralyzed, for 1d6 rounds. Those of up to 7 HD save at -4 or flee for 1d6 rounds, and stronger beings save normally and flee for 1d4 rounds on a failure. It seldom chases runners, preferring whoever is frozen in place.

Those who stand their ground meet two heavy fists for 1d6+2 each. Every hit smears corrosive slime. Unless it is wiped off within an hour, cloth rots away entirely and armor loses 1 point of AC for every three hours afterward; chain mail (AC 5) left dirty becomes AC 6 three hours later, AC 7 after six, and so on. Cleaning halts the decay but does not restore lost AC. Magical clothing or armor gets an item save vs. acid.

Its deadliest weapon is its breath. Once per turn it swells to nearly double size and blows a 10-foot cloud of vivid green gas straight ahead. Anyone caught must save vs. poison or die, and survivors who stay in the cloud take 1d10 per round. The gas lasts 2d4 rounds and works by touch as well as breath, so holding one's breath is useless.

At will, one per round, it can use any of these at 10th level: sleep, produce flame, fly, trip, enlarge, misdirection, continual light and animate object. Once a day it can try to gate 1-4 night hags (25% success), which it dislikes because they demand payment. Only cold-wrought iron weapons hurt it, and even magical ones of that metal deal only base damage, without enchantment bonuses. It cannot be surprised on its home plane, but anything with ears or a nose always notices it coming.

These lurkers of a volcanic lower plane ambush travelers, minor fiends, petitioners and larvae, with no aim beyond causing pain. They haunt busy trails, the plane's few fetid springs and spots where portals open, attacking anything weaker than a greater mercenary fiend and sometimes even those. Nobody has ever seen two together, so whether they hate each other or simply respect boundaries is unknown. They are petty and spiteful, perhaps driven by an endless, gnawing hunger. Each claims a hunting ground with several ambush points and lairs, and while it cares nothing for treasure, the remains of countless victims pile up in its main den.

It eats anything weaker than itself without a thought for consequences, which hardly matters on a plane of burning mountains with no sane ecology. Some sages think vaporighu are the plane's foulness given flesh; others say they were once mercenary fiends who forgot how to leave.