Here is an overview of the game, via Nintendo.com:
The award-winning indie RPG, Undertale, expands the series with a brand new game…in a different world!
Deltarune: Chapter 1! Create your own avatar, meet strange friends, and jump into the darkness. By the way, this first chapter is FREE, so please feel free to check it out. Like, you don’t really need to read this or anything… You can just try it…
Watch the announcement trailer below.