something i have been wondering, whatever happened to swapfell gaster/riverperson? idk if i'm remembering it right but it was a different more void-y gaster that got karma to come to the void with him right?

oh thats definitely a point i'm gonna have to HEAVILY rework in the reboot bc it is VERY clunky and handwavy atm, i believe my idea for that at the time was a result of a scrapped concept where Slime Man Gaster was able to "possess" other, not-erased Gasters in different universes. I think I was gonna play with that a bit more with my Papyrus-Gaster AUs like Retro or Smiley, but it just didn't end up fitting into the story at all :/ In hindsight it really doesn't make sense for Slime Man's lore anyway lol

I also had an idea that it was Shadow Riverperson/Slime Man (they're the same person btw!) the entire time, just presenting as an older Gaster that looks close enough to Swapfell Gaster to be convincing. If this was the case though, Karma wouldn't have been able to see them!

if i were to try to explain it in a way that actually works now, I could say that Swapfell Gaster was the one to lead Karma to the core as somewhat of a "trade" to get Fluff back. I'll be so honest the dude is a piece of SHIT and definitely prefers one grandson over the other so this kinda thing wouldn't be entirely out of his wheelhouse.

By the time they actually got to the Core, he would've slipped away and let Slime Man handle the rest. They have a lot more influence over the Core than anywhere else, so they'd be able to bait Karma into the void all by themselves :)