Demons are a species of evil creatures from Hell. They are often depicted as slender, pitch black beings with pointed ears, tail, and occasionally wings.
Demons in Disenchantment are small, shadowy, winged creatures with large eyes, pointed ears and arrow-shaped tails. Many of them appear to have pronounced noses and protruding fangs. According to King Zög, they closely resemble cats enough that he often confuses Luci for a harmless feline with the ability to speak.
It is safe to assume that demonkind are inherently evil, as that is the most common nature of demons, and is the personality of Luci. They can be sent to do the bidding of magicians, or sorcerers - such as the Maruvian Enchantress and Emperor Cloyd - by way of a curse, or magical bond.
Demons aren't exactly immortal, or at least not invulnerable. Exorcists such as Big Jo are the sworn enemies of demons. He hunts them down, performs exorcisms on their hosts (if necessary), and terminates them "with extreme prejudice" in the volcano, Mt. Piping Hot Suckhole.