Fan Casting Undertale (Anime series)

Long ago, two races ruled over the Earth: Humans and Monsters. One day, a war broke out between the two races and after a long series of battle, the humans were victorious. The greatest of their magicians confined the monsters to the Underground with a magic barrier. However, this barrier is not perfect. Many years after the war, in 201X, a human child climbs Mount Ebott for their own mysterious reason. It is said that those who climb the mountain never return. The child discovers an enormous hole in the mountain and falls in. After that, the story begins.