Deltarune is a game by Undertale creator Toby Fox that was revealed as an unannounced surprise on October 31, 2018, with its first chapter released on that day and the second on September 17, 2021, each for free. It is planned to have several chapters.
While the game does feature appearances of characters from Undertale in similar roles, Toby Fox has stated that Deltarune has no connections with Undertale's story and is a separate continuity.