The world was like always filled with good and evil. But stories and fairytales about underwater kingdom were just a thing that your mother was telling you before going to sleep. Little did anyone knew... It was truth

Deep in ocean, there's a small portal, that leads to underwater kingdom, the kingdom that belongs to kind hearted mareman, the king and even more patient maremaid, Queen, Kagaya and Amane Ubuyashiki. They have 5 children, 4 daughters and the youngest is the only son.

Of course there's no such a thing as perfect world. In this kingdom, there's evil. A evil king of demons, Muzan Kibutsuji, who's a terrifyingly majestic mareman.

He has his servants, the 15, strongest demons also knew as 12 Kizuki. No one knows why 12 if there's 15 monsters. No one knows where they are. No one knows how they look or what are their powers.

No one also know for how long they were here. Some maremaids thinks that they lived since the world started and some thinks that they're just a fairytale just to scare youngest children.

One time, in Kamado family, a daughter of Kie, Nezuko, was swimming around the deep hole. She was holding the gifted from her neighbor pearl necklace. Some time passed, and she unfortunately accidentally dropped it in hole.

Nezuko was heart broken and upset. It seemed very clear that she won't forgive herself for dropping it until she have it back.

A while passed and she was watching a pitch black area. It was gone and she won't go there, because her mother always said "Don't go to that hole, you don't know if you'll be a pray of a demon.", so she was staying there and just looking at literal nothing with a sad face