Once abon a time - long ago

Your best friend - your favorite buddy

Fallen down - asgore

Ruins - castle

Heartache - boss monester

Sans - papyrus

Nyeh heh heh - megalovania

Shop song - the song that might play when you enter a shop

Snowdin - snowtown

Bonetrousle - fortunately

Waterfall - lavaland

Spear of Justice - axe of Justice

But the earth refuses to die - for the future

Battle against a true hero - battle against a true hero(I still have to think about this one)

Hotland - coldsea(Inspired by @vizsz)

Spider dance - fire alarm

Core - heart

Death by glamor - low beats

Power of neo - high beats

Undertale - before the story

The song that might play when you fight sans - the song that won't play when papyrus blocks your attack

Megalovania - unfortunately/broken promise(Inspired by underswap broken promise)/vs the savor of this world(depending on which genocide route you are taking)

Small shock - the queen

bergentrückung - المستقبل

Asgore - the fallen

You idiot - did you really think you above the consequences

Your best nightmare - karma (inspired by @acul_horse )

Finale - the last battle

He is playing piano - she is playing guitar