Massive walls of fire sweep across the screen from the sides or top, leaving only narrow lanes or rotating gaps for your soul to squeeze through.
at the start of the encounter. This forces the player to engage in combat to progress, even if they have not killed anyone else in the game. Undertale Wiki
Master the Mercy: The Ultimate Guide to the Asgore Pacifist Fight Simulator
What follows is a simulation of the King of Monsters’ final stand—a fight where the objective is not to survive his wrath, but to survive his sorrow.