Disclaimer!
Undertale belongs to tobyfox - in no way shape or form is this supposed to be cannon material. This is my own theory on their backstory, and some far fetched inclusion with gaster and all that.
I would recommend you play the real game first to catch all the little things, but it is not required to understand the story. There will be a page accessible at all times that explains the basics of the world and how I interpret things if you need it.
