i'm getting the feeling that the fountain of the bunker is far more stained than any of the others
to the point ralsei would not be able to follow the lightners inside without petrifying
...Is 'stained' a Deltarune Fantheory Term I'm not familiar with? I'm guessing you mean, like, the opposite of Castle Town's Grand Fountain being made from 'Pure' Darkness. Pure Darkness allows almost every sort of Darkner to exist in it, 'Stained' Darkness won't even allow a Pure Darkness Darkner like Ralsei to exist....
I mean, it's pretty likely that we'd get a new Playable Noelle Segment sooner or later, and the way Deltarune Gameplay works that probably means benching Ralsei for at least a little while. That would be a good way to do it (although the way things are shaping right now, it feels like the Shelter would have Big Important Revelations for both Noelle and Ralsei, I think?).
Plus the Shelter is already on some level the Thematic Opposite of Castle Town (two currently-active Dark Worlds of mysterious origins that have existed for an undetermined amount of time before the start of the story, one is the 'Home Base' of the Heroes, and the other seems to be of the Villain's), so it would make sense for it to be it's metaphysical opposite on some level. Castle Town is inviting and habitable by basically (almost) every sort of Darkner, the Shelter rejects them all...
The question is more... how and why would a Dark World become ""stained""? We don't even know the how and why Castle Town was made and thus it's hard to even begin to speculate on the mechanics of how exactly the Grand Fountain is 'Pure' either. So what the hell would make a Dark Foutain into the hypothetical opposite of that?