Demons (悪魔) are a race that lives in the demon realm: Ente Isla's red moon. They posses demonic magic and come in different forms. The demons used to be living in tribes and engage with other demon tribes when they encountered them, until Satan united the demons and created a monarchy with him as their King and four great demon generals and a minister under him.
Demons are a collection of various humanoid and bestial monsters that attacked Ente Isle almost conquering it hoping to gain demonic magic, which was running out in their realms until the humans finally united and under the leadership of the hero Emilia brutally crushed them. They are greatly feared by Ente Islens. Unbeknownst to many denizens of Ente Isle, they come from the Red Moon. All demons need demonic magic to survive and gain their powers from, which is caused by fear and despair and possible other negative emotions they feed on. Powerful demons do not need to eat to survive surviving solely on demonic magic. Whilst weaker demons need to eat to survive. However, as shown by demon general Alciel and Devil King Satan, weaker demons can grow stronger over time.
It is unknown whether demons need a continuous amount of demonic magic to keep their immortality. For most of the time and unknown to many denizens in Ente isle, they lived in a tribal society that would constantly war on each other and hardly knew of the existence of humanity. It was only until relatively recently have they been united, organized, and lead under a single monarchy lead by king Satan did they knew this. Currently a system of capitalism and currency does not exist in the demon realms although their current king is interested in it in hopes that it would replenish the demonic magic that was running out in their realms. The demons were created by Ignora. They initially were humans from Ente Isle who were reject by the sephira, taken to the red moon, and experimented on turning into monsters called demons.
Demons appear to have a heavily patriotic culture where adult male demons hold predominate power and rule politically in government. Female demons apparently are never sent out to fight and stay in the demon realms unlike the male. Unlike what many Ente Islans think, demon children are conceived and presumably taken care by parents. It is mentioned by Ciriato that demons think it's noble to fight until their bodies are torn asunder and heavily discourages the concept of retreating. They are ruled by a unification of over a hundred tribes with powerful ones becoming chieftains.