Demons and humans live in two different worlds that are bound together, because both of the worlds are so small and their worlds’ principals are so similar. Demons and humans never got along, to say the least. They had many wars and fought on many fronts, but eventually their conflicts were settled by demon and human magic working together to stop the two planets from converging.
Before the enchantments keeping the worlds apart were created, the humans destroyed the demons’ way of reproduction. Demons were- and are still- somewhat similar to misquotes in the way their young mature. Humans destroyed the pools of enchanted water that the young grew in, thus cutting the population in half. The demons were never able to create those pools again, so they had to turn to another method, lest their species be wiped out for good.
First, they tried to create and nurture young in mercury, a common- and non toxic to demons- substance on their planet. Any young demon put to nurture in a mercury pool ended up dead, as did any young demon not put into any pool at all. Demons always have a few backup plans, though. They still had one more option.
Demons had always been able to entice a human into becoming demons by preying on their qualities, and nurturing one until it becomes dominant. After a quality becomes dominant, the demon preforms a sort of “rite of passage” ceremony that completes the transformation.
So, when the demons and humans created the enchantments to keep their plants away from each other, the demons made a passage. The passage was only accessible through the use of demon magic, and was hidden well enough that no human noticed. Every few years, a few demons go through to change a certain amount of humans into demons to keep the population healthy.
Humans generally ignore the existence of demons completely in daily life, as their time spent with demons was often bloody and deemed better forgotten. If demons are brought up in human conversation, it is generally in a mocking way. Schools are an exception to this rule, as demons are covered often in many topics, usually in a negative light. There have been some humans though, who have noticed demons on the human world, and brought it to the Council, the main governing body for humans. Many teams of experts are investigating the connection point, but have yet to find anything of note.