One of each thing we need to come up with instead of all the ideas I have, because I'm tired and need to get up in 7 hours so wheeeeeeeeeee. The design and demon/hq don't have to be paired and aren't written to be (unless votes go that way). I just like the theme.
Demon + HQ Idea
The Forgotten City - The Forgotten City rests within a vast jungle interspersed with drastic, steep mountains. The city was carved out of the peak of one of these very mountains, the capital of an ancient empire. The natives worshiped patently false gods, throwing vast hoards of material goods they deemed important into a nearby magma tube as "offering", as well as hanging chosen humans upside-down, cutting the body in a number of places, and exsanguinating the sacrifice atop a pyramid making up the highest point in the city (with the altar being the original peak of the mountain.). Sacrifices were performed daily, with suitable candidates brought in from villages and towns all across their land. Members of noble families were often volunteered by relatives for the honor of appeasing the gods with their most suitable blood. Nobody can be sure how many volunteers actually went willingly, and how many sacrifices led to a relative's sudden good fortune (in the form of inheritance).
Their behavior, and the bloodletting and nobile backstabbing (technically throatslitting) in particular, made The Forgotten City a particularly enticing location for Magoc the Many-Tongued, The Manipulator, That Which Twists, Bearer of Secrets, to start on it's path to dominance over all within reach once the entirety of their people were wiped out by a plague. Some suspect another Demon may had been involved but this occurred eons ago, so who knows. Either way, the jungle is littered with ruins and relics of this lost empire, and none yet have probed the jungle deep enough to uncover the capital at The Forgotten City. Except for Magoc, of course. [Terrain features I'm aiming for are Jungle, Mountain, and Ruin, if at all possible depending on what NUKE has planned]
Magoc the Many-Tongued is a muscular humanoid biped with a deep blue, almost black hide. It has a pair of curled horns and a snout. It has a wide maw that can open from ear to ear if Magoc desires, and seems to chatter incessantly but makes no noise. This stops when Magoc speaks, though the actual motions of It's mouth do not usually match the words spoken. Magoc the Many-Tongued speaks in many languages, with no consecutive words ever being rooted in the same language. Somehow though It's words are understood by those It addresses, even when speaking languages forever lost to time. It is a charming Demon, and more intellectual than most of It's primal kin. Magoc the Many-Tongued wishes to learn how to twist those It asserts dominion over so as to make their form (both body and mind) fit their function, and is very skilled at convincing others to assert said dominion for It.
Within weeks of rising in The Forgotten City, Magoc the Many-Tongued gained a large following. All was going well until the Angel made It's presence known. The Angel would no doubt seek to end Magoc the Many-Tongued's quest before it truly had the chance to start, and with the area of The Forgotten City under It's control, it was time to strike.
[I imagine most of that fluff won't have a direct impact. I was just in the mood to paint a picture. With words.]
Design Idea:
Cultist Conversion Chamber - The Focused Effort Thing That Does The Two Actions For One Thing Thing: An old stone chamber lined with shackles and caked in layers of blood centuries old, if not older. In the center lay a table with two pairs of attached shackles and a leather strap at the middle. The stone floor is laid out in such a way that the grooves between individual stones form a glyph. When the grooves fill with blood they allow a Ritual of Mutation to take place. The blood evaporates rapidly as the ritual takes place, and while a single human sacrifice is enough to fuel a very minor mutation, more complex mutations will require a greater number of sacrifices - hence the multiple sets of shackles along the walls. The target of the ritual is secured to the central pedestal and will likely be in some level of pain for the duration. Currently we can only give those who follow us the ability to see in little to no light, but we are sure to entertain some of our more...exotic ideas if we dedicate time and effort to the art of mutation.
[So I think this has four things it tries to do: give use a ritual for mutating, give us a dedicated building for a ritual of mutating, give us a mutation, and perhaps make it easier to perform or create mutation-related things so long as the use of the chamber is applicable. With the doubley designey thing it'd hopefully be Very Hard at most, but that's for NUKE to have final say on]