Demon

Introduction
Articulate monsters, who have been at war with every other race in the world. Demons are naturally proficient in magic. They devote the majority of their lives to one magic that they specialize in and master it. Like other monsters, they are individualistic creatures and do not have any concept of familial relationships. However, demons do maintain a basic level of interaction with one another in order to fight against mankind. Their societal hierarchies are based on strength, which can be assessed at a glance through reading a demon's mana. Because mana to demons is what status and assets are to humans, demons cannot imagine constantly limiting their mana, as it is akin to their very dignity. They in turn expect their enemies to not hide theirs, either. Like all other monsters, demons turn into particles of mana and disappear when they die. They have a similar lifespan as elves and view the passage of time similarly.

Appearance
Demons are monsters who evolved to look like humans to lure them. Demons all have two horns on their head, though the appearance of the horns varies between demons.

Personality
Demons are defined as articulate monsters by the Great Mage Flamme. They are incompatible with other races. Demons are by their very nature liars and will talk to humans to manipulate them into dropping their guard and sparing them, though they only know how to use words without knowing its meaning. Aura, for example, feigned a peace mission to destroy Graf Granat's Domain, and a young demon girl would cry for "mother" whenever cornered in an attempt to gather pity from humans that would kill her otherwise. While demons have been shown to feel emotions, they do not feel the same emotions as humans do, and as a result, are incompatible with human ideas of morality. Demons look down on humans and only view them as prey. Demons mock human empathy and think they are weak, which is why they constantly underestimate mankind. They are deeply proud of their mana levels, and due to the power-based hierarchies they form, they cannot conceal their mana nor do they have the intention of doing so.

Demon King's Castle
The Demon King's castle is located at the "northernmost tip" of the continent, Ende. It is also the alleged location of Aureole, a place regarded as "heaven" or the afterlife "where souls rest". It is an insanely cold place. According to Eisen, "much has happened there", alluding to events as of yet unrevealed in the manga.

Frieren found a Shadow Dragon's horn in the Demon King's castle and left it in Himmel's possession. Himmel thought it gave off an ominous aura. He kept it in a drawer in his home for the next 50 years, safekeeping it for Frieren.