Greetings

The architecture of the Infernal Demons could be inspired by fantastic Middle East in the style of Aladdin or the old movies about Simbad the Sailor, but in darker tones.

The environment could be predominantly rocky, with rivers of lava and deserts of dark sand.

See you

I honestly hope we can get something akin to the entirety of Blackrock mountain in WoW Classic. It was probably the most exciting 'hub' for all kinds of content, epic questing, giant sprawling laberynthian dungeons and a couple of raids.

The mere thought of a couple of friends exploring deeper and deeper through such a place for a whole night excites me!

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I suggest being in a desert with several oases because it gives a greater contrast to the lands of the other races, I have no problem with having a volcano or a river of lava, but please don't put lava everywhere because it can get a little tiresome, it would be nice to see a contrast.

Regions

-borders Maybe there could be something like a lava watershed that the demons take advantage of to make their cities more protected, they used lava rivers as lava filled ditches to protect their border cities. In the border region there could be trees with red foliage. The demons' border is more vivid because it is located on the outskirts of the desert.

-Interior The region that divides the interior of the border is a desert. In many cases, this desert ends up acting as a kind of natural defense against invaders.

After the desert one can begin to find large oases with rivers and springs that demons take advantage of. Some cities are designed underground because of the underground rivers, but also because of the heat of the desert. 

The demons' architecture can look something like this: