You're referring to Undertale, the critically acclaimed role-playing game developed by Toby Fox!

The scientist's experiments went awry, and the Underground became a battleground. The humans, fearing the monsters, tried to eliminate them. The monsters, defending themselves, fought back. And so, the eternal conflict began.

Sans, the laid-back skeleton, and Papyrus, his enthusiastic brother, were once part of a group of monsters who tried to make contact with humans. They built a barrier, known as the "RUINS," to prevent humans from entering the Underground, but some humans, like a certain scientist, were determined to explore this new world.

As it turns out, the Underground was once a thriving civilization, separate from the human world above. The monsters lived in harmony with one another, building sprawling cities and cultivating their own culture. However, as time passed, humans began to discover the existence of the Underground, and their curiosity led to... well, let's just say, a complicated history.

Here's an interesting story related to Undertale: