Crystal demons? Wild. Honestly, the lore varies so much it's almost a subgenre all its own, but the most consistent power I've seen involves light manipulation. They're not just shiny goblins. In a lot of stories, their bodies are literal prisms, so they can bend light to create illusions, blinding flashes, or even focus sunlight into deadly lasers. It makes for some insane fight scenes where the hero has to fight mirages or navigate a hall of mirrors made of light.
They also often have a defensive power tied to their crystalline nature. Shatter them, and they just reform from the pieces, or maybe each shard grows into a new, smaller demon. That's a classic horror twist. I read one book where the demon's heart was a single, perfect gem, and the only way to kill it was to find that core amid all the fractured reflections. Super frustrating for the protagonist, but super cool to read.
Beyond the obvious light stuff, I've seen them linked to geomancy or storing memories/knowledge in their crystal lattices. Makes them ancient librarians or record-keepers of forbidden magic. Their powers feel less about raw destruction and more about perception, refraction, and impossible geometry. That's what makes them stand out from your standard fire-and-brimstone infernals.