Demons are a powerful humanoid race of monsters in Goblin Slayer.
Demons are the common enemies of all other Prayer characters, and are commonly involved with the Demon Lord and/or his affiliates.
Lesser demons are aerial combatants that possess batlike wings and curved claws. They may take to the skies and dive down to make an attack with their extremely sharp claws, or may cast magic (such as firebolt) upon their foes. They are very nimble in the skies, making it difficult for even an elven archer to shoot one down. Despite being lesser, they are still demons, thus are still quite strong and not to be underestimated. 
Greater demons are usually depicted as massive muscular creatures with blue-black skin that possess enormous amounts of magical power, however, other such variations have been seen, such as ones with spider legs. Such is their magical power that they cannot be silenced by any but the most experienced casters.  Even if one were to somehow silence their magic, their physical abilities still pose quite a threat, with fists as large as a giant's able to punch through walls.  They make meals of humans they kill, whether they be men or women.