Update: The tease from the Undertale account now points to a site called Delta Rune, complete with a downloadable "survey program" that seems to be a fully featured game in its own right. You can only ...

Undertale's developer has said work on Deltarune is "going well." Taking to Twitter yesterday on May 17, famed Undertale developer Toby Fox shared a few snippets of concept art from his own hand.