"And obviously YOU wouldn't let it happen"
so im not huge on making theories. i usually just believe whatever other people say if they sound good or logical and hope for the best. but now that chapter 5 is so near, ive been thinking about this a lot. about what this game could look like in the future
now, before i go in-depth, i'd like to state my theory outright: The Final Tragedy involves Ralsei turning to stone in some way. I'm not sure about the exact wording of it, or if it's just a side effect of some tragedy far greater, but I have a hunch it might happen at the very end of the game.
It's been established since chapter 2 that darkners will turn to stone if they do not belong in a specific dark world. What this exactly meant was debated for a long while, some people thought it had to do with what dark fountain they were created from, and Ralsei was the only exception since, coming from a fountain of pure darkness, was also pure darkness himself and could travel to any dark world freely, which is a fine enough explanation on it's own.
Except, there's been instances of darkners turning to stone in their own dark worlds, being completely fine in places other than their own for extensive periods of time, etc. So, what really decides if a darkner belongs?
Purpose is the most popular conclusion currently. If the darkner is somehow useful or connected to the will of the fountain, or rather the person who made the fountain, they'll stay as is. If not, they turn to stone.
So even if the original explanation for Ralsei being unaffected holds up, this revelation brings up an entirely different possibility: that he can only exist as the hero described in the prophecy, which is why he can't turn to stone in the dark worlds we've seen. So far, we know three people that have made them: the Knight, Kris, and Susie, all of which Ralsei is an incredibly relevant person to.
Kris and Susie, the other two heroes, require his help and guidance. The Knight is destined to cross paths with him and clash with him again and again. He has a role to serve no matter where he goes. But if the Prophecy were to be fulfilled, what's left for him? What does he do then?
He could stay in Castle Town and still be friends with Kris and Susie, they could visit him every once in a while. Is that really enough? There's no way of knowing, I don't think. Especially not with how much Ralsei has reduced himself to the role of the third hero in the first place
Most of this theory lacks any concrete evidence, because really there's no way of knowing how exactly the ending of this game will go. But the reason why I wholeheartedly believe in this theory, is how Susie frames her reaction to the panel we didn't see:
"I wouldn't let it happen… Kris wouldn't let it happen…"
"And obviously YOU wouldn't let it happen."
She says this as if the final tragedy is about something deeply personal to all three of them. I see a few different theories about the tragedy that seem to neglect this part, other than sealing the grand fountain/another fun gang member dying. But outside of the former I see even less talk about how Susie emphasizes Ralsei here. As if Ralsei, out of everyone, would have the most to lose from it and would be fully prepared to stop it.
And of course, she mentions herself and Kris first. Why would Ralsei turn to stone if Kris and Susie aren't letting go any time soon? ...Is what I believe her internal justification is.
Another thing that isn't proper reasoning or evidence that I still wanted to say: For a while the triangles on the delta rune confused me. In Undertale they all represent monsterkind, and the upside down one didn't really mean much at all. In Deltarune, however, the triangles are very much the three heroes facing the Angel… or is it two? Since that one triangle is facing AWAY from the angel, would it mean that they aren't fighting at all? Well, what if it was Ralsei unable to fight?
