Hi!

I'm not Gaster anymore! Just regular ol' cy, here to explain what exactly all this "Auldentre" business is about.

I guess on a technical level, this is a swap AU for Undertale. I call it that because it makes organization and writing easier. However, from a storytelling perspective, Auldentre is its' own story, disconnected from Undertale entirely.

In fact, Undertale doesn't exist at all in this universe! _{Deltarune is still Deltarune, though. Unchanged. Mostly.}

The "Underground" in Auldentre is a simulated* world that exists within The Depths alongside the vessel, HERO (and potentially other familiar faces who may reside in The Depths ;3).

_{*When I say "simulated" I don't mean on a technological level, but more like... a play, I guess? It's kinda Truman Show-ish for HERO in the sense that, to them, the Underground is the real world with real stakes, whereas others know that it's all a smokescreen.}

For convenience purposes, the
list of swaps is as follows:

Frisk -> Vessel (HERO)
Asriel <> Asgore
Chara <> Toriel
Napstablook (N/A) -> Mad Mew Mew (Vicky) -> Mettaton (Gracidea) ->
Sans (Courier) <> Alphys (Aleph)
Papyrus (Garamond) <> Undyne (Melia)

But how DID we get here?

After the GONER_MAKER section of Deltarune, the vessel that supposedly gets "discarded" is actually brought into The Depths.

Upon seeing that the vessel has been, for lack of better terminology, cucked out of getting the SOUL by the second voice, Gaster decides to prepare it so that one day it can reclaim "what should've belonged to it all along," in his mind.

He does this by constructing the Underground as a "training grounds" for the vessel to get stronger.

So basically, chronologically, Auldentre takes place alongside Deltarune, sort of acting as an inverse to its' story. This is also sorta why I decided to split it into chapters (not to imply that they're 1:1 correlated).