Deltarune

Delatrune is a follow-up game to Undertale that takes place on an alternate timeline. Released on Halloween of 2018, Deltarune has only one chapter at the time, but a second one is planned without a release date.

Setting
The story in which Deltarune is told is separate from the one Undertale's is. As stated by Toby Fox, the game's pretty much everything, the two worlds are only similar. It's unknown if the two games are in alternate dimensions, but it can be assumed that they are.