('The Legend', A, B, and C, as heard in The Legend )
'The Legend' is a fairly important and complex motif that appears primarily in Deltarune Chapter 1.
Mirroring 'Once Upon a Time's introduction in Undertale's intro, 'The Legend' first appears in Deltarune's intro, which is the legend cinematic.
Outside of a few 'cameo' appearances in Ralsei's 'Lullaby' ACT and King's Chaos King, the motif almost exclusively appears in relation to Castle Town.