The legend of the mountain was true, but only half of it were given. The half never revealed, until then.
The fall to the underground will create a variety of versions of you-- classic was the original. Anyone who had never fallen, will never have versions of themselves.
It was like the surface was its own timeline, that nothing repeats and nothing stops.
But the underground is different, the fall will cause yourself to split to a million versions of yourself. Previous humans who fell became monsters, but only those with potential will be able to shine out as the superior being-- those who are true to their traits. There aren't just 7 humans who have fallen. That wasn't true. There were more.
The first to fall was the one who suffered the most, facing his own consequences and the others. The first to fall remembers the pain. The first to fall knows the truth. The first to fall was none other than Sans. The moment he fell, he knew about the anomaly. He knew about it all. But the thing about Sans was that when he fell in a different universe, it wasn't Sans, it was a different person. He held a different name. It was as if the split caused a different person to fall instead of him. After all, he was the first to fall. Then followed by others.
Decieved by his own desires to go back to his previous life, he wished to collect all souls, using them to bring himself back. But as he set on a lone journey, he never expect himself to encounter such a fate.