When you don’t have a really good name for it, call it what it is. And these are demons.
The original idea for these dark and dangerous spirits goes back to when I first read about Daedra in Morrowind, without really knowing anything about them before. The description that the elves called the gods “our ancestors” and those other beings “not our ancestors” was really evocative and made a big impression on me. I only actually met any daedra many years later when giving Skyrim a shot, and it turned out that they are actually just very generic demons. They are described as strange immortal spirits with alien minds, but in practice they are really just lizardmen or humanoids who do evil for the lulz. That was really lame.
I was reminded of that image I had had when encountering demons in Dragon Age, which in that world are simply called demons. They are somewhat more abstract, but I found them still not nearly as weird and alien and horrific as they could have been.
The third source of inspiration for what I want demons to be like, comes from Mass Effect. In the first and second game they are woderfully strange and terrifying, and not being able to do something proper with that buildup is one of the many disappointing flaws of the third game. (I’m still disappointed.)
“A god – a real god – is a verb. Not some old man with magic powers. It’s a force. It warps reality just by being there. It doesn’t have to want to. It doesn’t have to think about it. It just does.”
When I decided that I want to run my Green Sun campaign in D&D, I entertained the idea of using yugoloths as the dangerous alien spirits of the underworld. But then, how do you make them actually weird? Ultroloths may have some potential, but the rest are just too much regular humanoid enemies. They really can’t stand up to this lineup, so I might actually discard them entirely.
Working with images has always worked quite well for me. It makes it easy to notice the common elements between various different things you think have a somewhat general style. In this case, I think it’s quite obvious. The common characteristics of these beings are associations with darkness and shadows, to the point of not having any really discernable shape. They are more vague impressions of beings than truly physcial beings, combined with a lack of humanoid faces. I can work with that.
I don’t really remember where the idea originally came from, but several years back I had made the decision that in my fantasy, I want environments to be awesome. This means things that are huge, but in the modern world we are used to cities that stretch to the horizon and buildings with hundreds of rooms. But what still impresses us ars things that are really tall. And here I admit taking an idea directly from Tolkien. That guy put massive towers everywhere in Middle-Earth. Because giant towers are awesome. Why make a dungeon underground when you can have it in a tower?
Masks are fascinating objects. The first thing that comes to my mind when I think simply of “mask”, is something that obscures the face. But specific masks also replace the wearer’s appearance with an alternative face. And in addition to that, many types of masks also serve to protect the face. They represent a barrier between the self and the outside world more than any other kind of armor does. In an antiquity, and to some extend medieval cultures, we encounter masks both with priests and shamans as part of rituals to contact the gods, and also with warriors as part of the helmet, the most critical piece of armor after the shield, often displaying a fearsome appearance to the enemy. I think everyone will immediately agree that masks are extremely complex cultural objects with deep and layered psychological meaning when you start thinking about them.
Once I had noticed that I really love the look of masks on character images, I went back to see if there are other common motives in the reference pictures I had collected. And a less striking but still noticable pattern was the frequent appearance of unusually colored lamps. The use of striking lighting in visual arts has long fascinated made. Primarily in natural environments, but also the highly stylized urban environments of 
Unlike plants, fungi don’t use photosynthesis. Instead they get their energy to grow frm, the chemical energy released in the breaking down of organic compounds. They digest the remains from other living things just like animals do, which allows them to live in darkness, subsisting on organic material brought in by animals or wind and running water.
As a fan of Robert Howard, I of course love big giant snakes. What for Lovecraft was his fish and for Tolkien is spiders, was for Howard his snakes. It’s impossible to imagine ancient fantasy or Sword & Sorcery without giant snakes. Snake men are also super cool. But then there’s also giant centipedes. And giant eels. Giant grubs and caterpilars, And on the mammal side, I think the coolest group of predators are weasels. Which also have a generally similar body shape. And then there are various river monsters around the world that occasionally have been interpreted by artists as giant monstrous otters. And damn, I do love carrion crawlers.
It’s always a good idea to step back and consider what you’ve been doing all the time was really the way to go. I had started with Green Sun with the intention to fully leave behind all the Dark Fantasy and Weird Horror stuff and try to make a pure Bronze Age Dinosaur World setting that I had envisoned several times in the past, that also absolutely isn’t D&D in any way, shape, or form. But three months into this, I find myself noticing that I am really missing some great influences that greatly impacted me in the past and my perception of what makes meaningful stories and fantastic worlds. I simply can’t go on trying to keep out any influences from Dark Souls and The Witcher 2, and there’s actually lots of great stylistic elements in Baldur’s Gate II, even though it’s very much an urban setting.