On October 30, 2018, Undertale creator Toby Fox lit up Twitter with some serious teasing, more or less informing fans that he had something new up his sleeve. He wanted to share it with everyone and perhaps get some feedback along the way, supposedly. Instead, Fox dropped the new game/proof of concept/whatever it is on Halloween 2018 and it’s called Deltarune.
You can download Deltarune for free via its official website, and it’s a totally playable game that is definitely related to Undertale. It even goes to the trouble of incorporating characters that were in the original game and building upon its hybrid RPG/shoot ‘em up gameplay mechanics.
Much of the current speculation is just how related it is to Undertale, but most fans are trying to respect Fox asking for a 24-hour grace period before all the crazy spoiler talk hits the internet in full. So in the meantime, check it out for yourselves!
Source: Deltarune website