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.
Unlike angels demons do age, albeit at a much slower pace than humans. Some demons live for centuries and some have lived for millenniums with only the latter showing any signs of aging. It is unknown whether demons need a continuous amount of demonic magic to keep their nigh immortality. Many demons mature at a slower rate than humans though. As shown with Satan who while still being a child apparently lived for decades. 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. 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. Before Satan united them, strength was the most valuable attribute to them. They rarely retreated from battle and considered victory not on the cost of it but whether they succeeded in completely destroying their enemies. When fighting they would try to not just stop but annihilate opposing clans killing everyone they can without mercy in fear that the survivors would grow and attack them possibly destroying their clan. Unlike what many Ente Islans think, demon children are conceived and presumably taken care by parents.They are ruled by a unification of over a hundred tribes with powerful ones becoming chieftains.
Many tribes such as the goblin, giant, black goat tribe were mentioned in the series
Malebranche: Perhaps the most powerful and one of the most numerous tribe of demons skilled in necromancy, illusions, flight, and psychological warfare.
Ashen Horns: A powerful tribe of of demons who have the likeliness of a Minotaur. They are famed for their heavy build giving them their monstrous strength as well as their skills in precision magic
Iron Scorpions: A tribe even more powerful than the Ashen Horn tribe known for their very tough skin that naturally almost encapsulates their body, defensive magic, and powerful telekinetic powers capable of throwing gnarled boulders at far distances to kill their enemies. Before the Devil King united the demons, they were the only tribe of demons who would retreat from battle when it proved better to do so and fought much like how human nations fought over nations and not like almost mindless horde of monsters. Their skin is still far from invulnerable as typical soldiers are said to die from Lucifer's magic bolts.
Black Goat: It is the tribe the Devil King came from, known for being goblin-like and having horns and legs similar to goats. Contrary to what many Entle-Islens may think, it is a very weak tribe as it was annihilated by a single large muscle bound demon who could not use a single spell and was as dumb as a brick. The survivors all died from a conflict from another rival tribe leaving only the Devil King the only surviving member of it. It it mentioned that the Devil Kings's size in his demon form is quite large among members of his tribe. In terms of strength they seemed to have as much strength as an average Entle-Islen civilian, albeit with a few having the ability to fly