"My name is Asriel Dreemurr. Six years have passed, and we made it to the Surface. But the world around us is much more dangerous and complex than we initially thought. To reach true reconcilation between Humans and Monsters, we need to help improve the hurt world around us. It will be a difficult struggle for me and my loved ones...but it will all be worth it in the end."

You "fall" down a hole. The outcome isn't quite what you thought it would be. (Pre-canon. Notes and content warnings inside.)