Me and my girlfriend have been playing Deltarune together and I have had inspiration for a theory that I sadly haven't seen around much.
I would like to believe, until reasonable evidence to the contrary is given, that the three heroes of prophecy are not, or perhaps once were not, our main characters of Kris, Susie, and Ralsei. I think that the three heroes of prophecy showed themselves in the past already. I believe the Roaring Knight, Dess and Asgore were the first heroes. The Roaring Knight is the human, Dess fills the position of the monster from the prophecy.
Asgore, I admit, is a bit of a stretch. Yes, he is also a monster and certainly not a darkner like Ralsei, but I don't believe it is ever mentioned that is is a darkner hero. Only that they are from the dark. Assuming that I am correct, then it could simply be revealed at some point that Asgore was the first of the three to discover the dark world or something of the sort. And as an aside, we never really hear the proper prophecy as far as I know. Ralsei tells his rendition of it, sure, but those are just his words and interpretations of the prophecy, perhaps even just little white lies he tells to drive home the point. For example, it may not be that a PRINCE from the darkness is one of the heroes, but he just interprets it that way because that fits the narrative of their journey best. This could also apply to the whole "from the dark" thing. On the subject of the whole prince part, if it is in the prophecy, it could just mean that the younger Asgore was more of a prince, or saw himself as a prince in the dark world. Either way, Ralsei being an obvious stand-in or version of Asriel, the son of Asgore, just makes it easy to see on the surface how he could fill the role Ralsei does.
Now, in honesty, I don't have anything much to do with this theory of mine. If it's true somehow, it may just be used for a story purpose I cannot think of right now. Maybe they discovered the prophecy and saw the panel that Susie destroyed, which lead to perhaps an accident or even knowing decision to kill Dess. Maybe the Roaring Knight was addicted or consumed by the dark world, such that they changed to an unrecognizable version of themselves. Or the prophecy called for some suicide pact and the dark world brought them back.
There are a lot of ideas that could be made with this. But, I just really like the theory. This idea that the prophecy, the very story of what is supposed to be our own main characters as the heroes, is turned upside down on its head. The first major plot point we ever hear of is misunderstood. I dunno, seems cool to me.
And yes, I know we have the lightner religion's interpretations of the prophecy and stuff, but that's still all a perceived version of it and from the light world perspective, I don't worry that it somehow breaks my theory.