Wow, you don't know?
It's a sequel. Like, a full-on sequel. A downloadable and playable demo containing the first chapter was officially released by the creator on Halloween, hence why everyone's freaking out.
https://www.deltarune.com/.

Like Crowley said… except more complicated.
This game happens in a "Elseworld" setting, as it doesn't fit the Undertale canon at all despite reusing characters from it (They're on the surface, but Asriel is alive (If off-screen), Alphys and Undyne don't know each other (And the latter has both eyes), Catty and Bratty hate each other, Gerson's dead, Frisk is nowhere to be seen, among other stuff. Toriel still divorced Asgore tho), and the majority of it happens in a Dark World where you don't see any of them anyways.

Also, the game maintains some characteristics from Undertale, but is also different. All 3 characters (Kris the Human, Susie the Monster (AKA the purple one), and Ralsei the Dark Prince (AKA the green one)) are playable in a more traditional RPG, even if Mercy's still there. But Mercy's is irrelevant. Remember how "Choices" was a mayor subject on UT? Well, in Deltarune, to quote, "The choices you make don't matter". Use your love, use your hate, you aren't allowed, to defy fate.

Oh, and it is episodic: We just got chapter 1.