Personally, in this category falls the myriad of Greater Demons and the Hells. I have never ran or played in a satisfactory foray into the Hells or even found a GM (even me) who is willing to use the Greater Demon’s in any manner other than as shadowy figures out there five or six layers away from whatever intrigue or action was roiling around. Bringing them in closer is tantamount to death for any group and can become a campaign buster if not handled well. So how to work it?
Well, I had in the past put together a fairly challenging module into the Hells, but even so kept the Greater Demon (Thangumokk) in question away from the party directly, by giving them ample opportunity to avoid given some intelligence and strategy. The entire theme of the adventure was the terminus of a cycle involving a cat made of jewels and amber, glitteringly beautiful and animated; wings of sparkling, begemmed silveel. It is rumored that where it strode or flew, the land would change growing vibrant and strong; that it was taken to Hell to forever deny this to those of need of the bounty.
I put together an interesting trip, allowing the options of venturing there via a few choice, magik artifacts that could cross the planes and move them to the twelfth Hell. Another potential was to Hell walk there way there, but it was the most dangerous venue of the all. If they actually made it there, hopefully they had prepared well enough for the burning rays, non-existent breathable atmosphere, glare of the foiled, reflected lands and jagged, blade like terrain. Not to mention swift moving electrical storms, weirdly changing gravity, pulsing radioactive clouds that hugged the group and omnipresent, falling meteors that cascaded down in showers, splintering the land more and give something for the Sun Demons to play with and in.
The Greater Demon kept his “pets” in a few places at “home”, most of them environmentally set up to allow his “pets” to survive. These were the targets, and what they sought could be in any of the five they knew of or anywhere else, since it could likely existence in this place.
It was an interesting ride; one I have ran only three times in the last decade or two. One group made it as far as the Hells but perished in an encounter with its “indigenous” populace.