Agatha All Along ending explained: Who dies, what happens at the end of the Witches' Road, and what happened to Nicholas Scratch?
Agatha, Billy, and Jen all set off for the last trial. But along the way, Agatha is stopped by Rio/Death, who says that Billy is an abomination because he stole a second life. However, if she simply tried to kill him, he'd be able to resurrect again, so he must go with her willingly.