What Deltarune's Dark Worlds and SCP-3930 Say About Reality and Fiction:
The way Deltarune's Dark Worlds function as living reflections generated by your mind when it's subjected to darkness so dark it becomes anti-light reminds me a lot of SCP-3930: The Pattern Screamer, and I think comparing the two provides some interesting insights:
SCP-3930 is described as a small gap in reality located in the Russian countryside, within which literally nothing exists. Since your brain can't comprehend non-existence, it uses the same pattern-recognition tricks it uses to see faces where none exist to fill in this gap with what it THINKS should be there, meaning when you look at the gap all you see is regular Russian wilderness that appears no different from the area surrounding it. The Dark Worlds work in a similar way, being created by your mind's misinterpretation of a mundane space when perceiving it through anti-light.
Like with the Dark Worlds' darkners, SCP-3930's mental projection effect can even create imaginary consciousnesses, since if you watch someone step into the gap, your mind adds them to its projection of what should be there even though in reality that person has ceased to exist. This results in a fascinating "exploration" log for the SCP, in which the "explorer" sent in to the gap gradually realizes they don't actually exist any more, and are just a projection of the researchers' minds speaking over a live radio. When this radio's frequency momentarily gets interrupted by another broadcast, the pattern is broken and takes the explorer's voice with it, leaving the researchers to realize they'd been talking to empty static the whole time. The Darkners (and Lightners in a meta sense) work the same way. They're imaginary entities that are brought to life within the minds of higher beings. Turn the lights back on or stop playing the game, and those imaginary projections return to our minds.
I think perhaps the point of Deltarune's story is going to be that just because these worlds are imaginary doesn't mean they aren't in some way "real," so-long as you're willing to continue creating them within your head. After all: the thing we call reality is already a controlled hallucination authored by our brains, and as far as your perception is concerned, the only difference between real people and a game's NPCs is the depth of interaction you can have with one compared to the other. Hell, I bet many of you are kinder to NPCs in games than you are to strangers online.
This idea of "realness" is further explored in Undertale's Genocide Route and Deltarune's Weird Route from another direction. The Genocide Route uses its mindless grinding to make the player gradually turn their brain off and stop seeing its characters as "real" until they're no different than the game's soulless antagonist. The process intentionally ruins the game within one's mind by permanently breaking its sense of "realness," while also serving as a good simulation of how people become desensitized to their own actions in real life. Deltarune's The Weird Route meanwhile goes in the opposite direction: instead of game-ifying the player's actions until their weight is lost, it has them commit acts of manipulation and abuse which feel so real that it becomes deeply uncomfortable, even though the victims of their actions don't actually exist. Sure, most people will still DO it, but the feelings of reluctance and guilt suggest that on some level we ARE perceiving our actions as amoral, which would only make sense if on some level we considered this imaginary world to be real.
You can tell us nothing's there, but we can't stop our minds from filling in those gaps, even if it means having one-sided conversations with someone that doesn't exist.
