Obviously my taste in demon illustrations is idiosyncratic, but, even so, when I think of demons, the ones that come immediately to mind are those I saw in the pages of SPI's DragonQuest as a younger man.
I'm still not entirely sure why this particular image affected me so. Looking at it now, it still creeps me out a little, especially the demon on the left, which reminds me a bit of a sinister Carnival mask. Maybe it's their mocking expressions that unnerve me; I don't know. As I've said before, though, I find demons like this far more frightening than I do those that just look like Big Monsters.