Sans disappears after you fought him, you passed the capital and into the last corridor, moonlight shines through the corridor, birds sounds are gone??, flowers blome and die as time goes by

You encountered sans once again

Sans: your journey is about to end kiddo

Sans: it's a beautiful day outside, birds are dying, flowers are rotting, on days like these, monsters like you, Should Be Burning In Hell

Your Very Soul Shines And Your filled With Determation