The Retriever ... vs. Demons Type I-VI

I see the adventurers have stolen the ostrich-egg-sized navel ruby from that demon statue on the 9th level. They're trucking toward the stairs, whistling and skipping, chatting about building a little pied-a-terre in the Merchants' Quarter with the proceeds. When they look back, what have you got coming at them, to make them well and truly befoul their breeches?

• A Type I-IV demon, all looking like some Thundercats villain action figure?
• A Type V? Okay, she's badass, but she has a human head. You can talk to her. Maybe throw her back the gem, and a couple of henchmen to sweeten the deal, and hope she backs off easy.
• A Type VI? Too overexposed. The man from Oxford called and he wants his fallen angel demon back.

Here boy! Fetch!

No, you want something really out of the ordinary ... mindless, uncaring, mutated, terrifying, Lovecraftian. Arachnid. Four scything arms of death.  Four eye rays. Dragon-breath-level damage from three of them. The fourth can turn you into stone, or lead, or a nice golden consolation prize, or mud, from which you can only rise if a mud to stone and stone to flesh spell happen to be handy within 10 minutes.

So good luck with that.

Or if you like any of those other demons? You can have them too. "Demons sometimes mount howdahs on the back of a retriever and ride on the creature to the hunt."  

Need I say more ... sometimes the Fiend Folio is better.