I MADE AN AU?
Or is it an AT? c;
(o wait i didn't FLASHI did ;^;)
Okay, here's how it goes.
The start of the story takes place soon after Toriel left Asgore behind, over the fact that both their children died and Asgore declared all humans falling down would be killed. Asgore was, obviously, very upset.
One day, Gaster's wife was going to have a meeting with Asgore. But Asgore didn't want to see her, and he told her to leave him alone. She refused, though, and continuously asked if she could help him, but he didn't want her help, and he threw his trident at her in an attempt to scare her away, but it hit her and she died.
Later, Gaster found out that his wife had died, and that Asgore did it. On his way home one day, he was crying near the core and fell in. Asgore was sorry about this, and decided to adopt Sans and Papyrus because they were very young, and they had no parents anymore. Sans and Papyrus grew up oblivious that Asgore wasn't their real father.
There have been many attempts on Asgore's life from certain Monsters that heard about how Asgore killed one person and caused the death of another.
After the first attempt on his life, Toriel came back and said sorry, and became Sans and Papyrus's 'mother'.