Undertale(2015)

A small child falls into the Underground, where monsters have long been banished by humans and are hunting every human that they find. The player controls the child as they try to make it back to the Surface through hostile environments, all the while engaging with a turn-based combat system with puzzle-solving and bullet hell elements, as well as other unconventional game mechanics.

"A long time ago, two races ruled peacefully over the Earth: HUMANS and MONSTERS. One day, a terrible war broke out between the two races. After a long battle, the humans were victorious. They sealed the monsters underground with a magical spell. In the year 201X, a small child scales Mt. Ebott. It is said that those who climb the mountain never return. Seeking refuge from the rainy weather, the child enters a cave and discovers an enormous hole. Moving closer to get a better look... the child falls in. Now, our story begins."