The prophecy was talking about Kris, Noelle, and Ralsei.
Dark world battles are the same as in Dragon Blazers, Noelle's favorite game, and the Knight? Is her long lost sister.(Inevitable mother 3 parallel)
Kris and Noelle ultimately are both the heroes and the lovers of the game, but unfortunately with the correct prophecy the world was going to end and become what we see in Undertale.
But here comes Susie, potentially someone that Gaster created in other to defy the fate, thus why one of the followers is directly talking about her.
Susie constantly fucks over the game, breaks the rules, and stand ups against control.
She will be both the hero that saves the world, and a villain who are going to stay in your way later down the line.
With Susie in, the lightworld will be saved thus preventing Undertale from happening, but for what to happen the darkworld will have to disappear for good.
That's when Susie becomes the villain, the dragon, for her the darkworld is everything, she will be the ulit sacrifice in order to achieve the happiest ending.
She will seal herself, either becoming an eternal part of dark world completely cutting herself from the lightworld, or becoming a seal.
Ultimate, I think the game will end with Kris and Noelle staying in light world, while Susie and Ralsei forever stay in the darkworld.