Do you believe that vampires, werewolves, elves and succubus exists? Well, let me tell you a tale. Whether you believe it or not, 'we' are certainly there, mixing in with the normal humans in their world since over thousands of years ago.
But before that, where were we? There are actually many realms besides the human realm. 'We' used to live in a peaceful place called the 'The Demon World'. From the name, it may sound negative, but it was a surprisingly tranquil place where people laughed and were full of smiles, until one day, a youthful demon who got bored of the peacefulness decided to visit another realm.
He took with him a few of his friends, and opened the forbidden gate that connected to other realms. He decided to go to the human realm as it seemed amusing, how people strived to live even when faced with predators. However, the youth and his friends didn't know that the moment they opened they went through the gates, a tragedy was soon to come.
The youth, wanting to be rendered as a hero, decided to use his knowledge from the demon world which had many resources similar to the human world and teach the humans how to make their lives better. He taught them how to create tools, hunt for preys, and many other things.
However, a demon and a human's life span were different. The demon was able to live on for over thousands of years, overseeing the humans. He felt like a king, having countless people under his rule. Little did he know, humans were constantly evolving and they had noticed that the demon's life span was strangely long. He was there in every generation. People started to fear him and from their fear, they decided to cause a riot, and interrogated the demons and his friends for their identity before killing them.
The humans now knew of the demon realm. After knowing of such a fearful place, of course, they decided to strike first before they were attacked. With the knowledge provided by the demon and their evolved brains, the humans were able to create a weapon that was able to kill demons easily. They decided to invade the demon realm.
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The humans did not go easy on them in fear of being retaliated later on. They killed many demons that were living near the gate. Soon, the whole demon world heard of the news and decided to counterattack the humans. To do this, they had to decide who was in charge, even though the reason that the place was so peaceful was because there wasn't any ranks and thus, no conflict.
The ranks were decided based on each species' magic power, making vampires, who were the most powerful at the top, being in charge of the last decision, making them become the monarch. After the vampires came the elves and mermaid, in charged of advising their newly appointed king. Next came the succubus/incubus. As they were able to produce unique weapons and were quite physically strong, they became knights that were in charge of protecting their world. The beastmen were in charge of taking care of the injured, as they were highly skilled in healing magic. The rest of the demons had either fled the demon world already or didn't have enough magic power to be given a rank.
After the ranks were formed, they immediately began a meeting on how to deal with the ruthless humans. They decided to use magic power, that humans weren't able to use to defeat them. The war didn't last long as the demons were clearly stronger, driving the humans back and trying to spread word that using the gate was a taboo in the human world to stop them from ever entering their territory ever again.
The humans believed the rumors and no one went near the gate ever again. Over time, the gate's existence was forgotten, as well as the existence of the demons. However, the humans were the only ones who didn't dare go near the gate. What about the demons? They were vengeful beings and couldn't forgive both the youth and the humans. They decided to invade the human realm quietly, waiting for a chance attack.
Years passed and the demons had caused many chaos in the human realm without the humans knowing. At first, it was fun for the demons to trick the humans, but over time, the demons became bored and decided not to take revenge but instead, blend in and live with the humans peacefully. Originally, demons were peaceful beings to begin with so they gave up the idea of revenge after a few thousand years. This leads us to the present, where demons blend in and live with the humans.