You were watching the battle between Frisk and Asgore from afar and Frisk won but spared Asgore. This was your moment to shine. You killed Asgore and absorbed his soul and the six human souls he was guarding. Whoever had seven human souls would become a god. If one was missing, it was Frisk's. You used the human souls and became an extremely powerful being. Frisk looks at you in fear.

"Uh my god."

They tries to escape using the save state but it doesn't work because you're in control now.