Undertale: Legends of the Region

A long time ago, the earth was equally ruled by two races, humans and monsters. One day, humans were worried that monsters would use the power of human souls to destroy humans, so they launched a war first, shattering the peace between the two sides. Under the almost massacre-style invasion, mankind won a complete victory. Humans drove the monster underground and set up a very powerful magic barrier to trap the monster underground under Mount Ibot. According to legend, none of the humans who climbed Mount Ibott came back alive. In 201X, a human child climbed up Mount Ibott and found a large hole. He accidentally tripped over a vine and fell into the hole... PS: This is the original plot of the game Undertale (slightly adapted)