('The Door', as heard in The Chase)
'The Door' is a seldom-used motif that, as discussed on the page 'On the Origins of Undertale and Deltarune's Leitmotifs', appears to have been lifted from an earlier Toby Fox song, Nightmare Knight (no relation).
(probably)
In any case, despite being used in so few songs, the motif is somewhat commonly brought up in lore discussion, primarily because of Nightmare Knight, which has led some to believe that the motif is related to the Knight them(it?)self.