In the same vein as what Galius says about structures based on a mountain, but what if said mountain is the remains of a huge creature? a giant or epic monster.
How about a bag of "wintering supplies", this would be a consumable/item that would unlock extra space in the inventory, where only food can be stored. Similar to the warlock's bag of soul gems in
They could get an exclusive mount that they would get through assassination, I mean, since demons are direct enemies of humans and they even hunt them, how about getting an exclusive mount for assi
How about, melted candles on the horns?, that would be great, imagine a demon warlock with this aesthetic.