This interview (www.nintendo.com/jp/topics/ar...) and excerpts from the Undertale Collector's Edition Sheet Music Booklet (www.reddit.com/r/Deltarune/...), state that the Undertale songs "Heartache", "sans.", "Nyeh Heh Heh!" and "Bonetrousle" were initially from Deltarune. #Undertale #Deltarune
