Monofell/Monotale is certainly quite an obscure Undertale AU

From what I could find out about it, the AU is where the Undertale characters are facing their inner struggles with some relation to a colour theory

aside from that we do have some designs of some of the characters and Sans here is one of them (obviously). Now I know the design looks rather similar to Outertale Sans's outfit and design but that's likely unintentional as I think the creator of it was inspired by outertale sans's design to make Monofell/Monotale Sans's design (I think I might make a sprite of the papyrus design too but we'll wait and see. Hope y'all like this)

Monofell/Monotale by PC-Doodle

Undertale and original sans Sprite by Toby Fox and Temmie Chang

Art and edits by Me