'Ruins' is initially heard as the theme of the Ruins, and is one of the most recognizable leitmotifs in Undertale (though it has yet to appear in Deltarune).
The motif is split into two relatively distinct parts, 'Ruins A' and 'Ruins B':
('Ruins A' and 'Ruins B', as heard in Ruins)
Together:
('Ruins' as heard in Ruins)
Notably, 'Ruins' almost exclusively appears alongside an ostinato or other backing melody, such as 'Another Medium','Undyne', or its own 'Ruins Ostinato':
(The 'Ruins Ostinato' as heard in Ruins)
Additionally, section A has an alternate rendition occasionally used on its own: