During the years that Gaster was alive as a skeleton so to speak he had worked a secret project a project that ended in his death or the erasing of his existence his sons unknowingly come across this experiment and at the same time the first human to ever fall has fell who ever said Chara was the first.

Just my spin on what happens after the 'I do's.' It's basically what I think Hazel does, it's not meant to be a masterpiece but I don't think it's too bad so give it a read and let me know what you think, or what you would change about it. Enjoy. ;)