>>528288958
They were supposed to lose to the Knight here, Ralsei must have seen in the prophecy. I think the extra struggle is a hiccup of fate granted Gaster straight up gives the player the power to keep trying. So if for a moment it seemed like the prophecy was about to go bust, Ralsei being terrified and relieved at the same time that Kris would go as far as knock down their own team as a way the prophecy goes on untouched is fitting.
This also goes to show getting the pure crystal will be critically important, I really don't think you were meant to have it in the prophecy.