It turns out Chester is searching for The Ripper, the tyrant at the top of Craft Castle. When he finds them, Chester nopes out at the last moment, allowing for the shelter squad's capture. When him and Carmen meet back up, Carmen confronts him, but neither of them have the will to attack.

Relevant swaps:
Vessel -> Susie (Carmen)
Jevil -> Lancer (Chester)

Chester is just a weird, meek elf man that loves fun and games. When his closest compatriot goes missing, he seeks out help from the shelter squad. Carmen goes with him cause he, admittedly, seems more trustworthy than Enello.

Now, the folks just call him Al, and he works as a shop keeper at the lowest rung of his own kingdom. Because of the angel's actions, he has a strong disdain against the graylites, but puts his prejudice aside in hopes that you can set things right.

King Albemarle was once the beloved ruler of the craft kingdom, but one day an angel descended and nothing quite worked right after that. His heir started acting strange and he was usurped by a mysterious figure known only as "The Ripper."

Relevant swaps:
Dess -> Kris
Vessel -> Susie (Carmen)
Friend -> Ralsei (Enello)
King -> Seam (King Albemarle)
Lancer -> Jevil (Prince Ribston)

Roots -> Abandoned Classroom (Craft Kingdom/Violet World)

Extra note for this that I forgot to add earlier:

The ghosts and dummies from Undertale swap, specifically Napstablook/Ruins Dummy and Mettaton/Mad Dummy.

These swaps will have more importance when it's time to draw Mad Tenna, but yea, that's why there's the silly little shadow Enello here lol.

Also, extra note:

Since the dark fountains in Deltarune are very "liquid," the inverse for that is the color worlds being more "plasmic." They play more into notions of energy and chaos, with their creation being more violent as they explode into any given space rather than slowly flowing in.

I don't have any songs made for Team Dark, I don't really have a lot of experience making music as it stands, but maybe Anything will exist in actual musical form eventually lol. For now... the titles/song names are there just for fun and exist to fuel the imagination.

All these drawings are titled after (theoretical) songs from Team Dark. A lot of Rude Buster (the song) replacements in swap AUs are named after... Rude Buster (the attack) replacements but I thought it would be fun to name it after an ACT instead.

Relevant swaps:
Dess -> Kris
Friend -> Ralsei (Enello)

Depths -> Castle Town (Wild Village/Rainbow World)

If too many holes are opened in the gray world, every color melds together and the gray world turns white. Everything becomes indistinct as the parallel worlds fall atop each other and the sky burns so bright it's blinding.

This hole will grant access into a color world depending on the energy of the creator. Someone who's passionate, fiery, and determined can grant access to the red world. Someone who's calm, melancholic, or has a lot of integrity can grant access to the blue world, etc etc.

Many worlds exist in the same space simultaneously, each with their own unique energy. In the center rests the gray world, which contains the energy of every other color world in trace amounts. If a being in the gray world channels their energy into a blunt object, they can bash a hole in reality.

Relevant swaps:
Dess -> Kris
Vessel -> Susie (Carmen)

Barrier -> Dark Fountains (World Mirrors)
Souls -> Dark Worlds (Color Worlds)
Shelter -> Supply Closet

Basically in Team Dark, it being an AU is a problem in-universe the same way Deltarune's "one ending" is. In ch4, Gaster is doing.... Gastery stuff in his eeeevil laboratory and the shelter squad have to come face-to-face with the alternate versions of theirselves.

Ngl one of my friends convinced me to play 999 cause they said the story was very "Deltarune-like" and when I got to the twist I was just like "no this is undertale lol, ever heard of narachara?"