My Favorite Video Game -- Undertale

There was a war between humans and monsters. Upon being victorious, humans trapped monsters under a mountain--Mt. Ebott--using magic. Years later, a child falls in. The king and queen care for them and their own son, the prince, as if they were siblings. Eventually, the child falls ill and dies the next day. Stricken with grief from losing his best friend, Prince Asriel takes them back up to the land of humans, using their soul to go through the barrier temporarily. The humans are afraid, thinking Asriel murdered the child, who was named Chara, and they attack him. He doesn't fight back and lets them attack him. Once done, he returns Chara and leaves to Ebott, but dies before returning to the palace. The king, Asgore, is outraged, and is looking for six souls to shatter the barrier and redeclare war. You are the seventh human, the last one, Frisk.