Apart from Undertale’s soundtrack and sense of humor, everything else is fresh.
Though some old characters do make an appearance, they’re only in supporting roles. The game is set in a different world, with another storyline and new characters. While most people assume Deltarune to be the prequel or sequel to Undertale, it is anything but.
It stayed that way until Toby Fox announced the release of Deltarune on Twitter. Undertale became a huge success, with a fandom that began participating in development.
The character that you chose had to find its way home, after falling into an underground and hidden monster world.
