JCoxeye's Leitmotif Index:
'Your Best Nightmare'


('Your Best Nightmare', as heard in Your Best Nightmare)

'Your Best Nightmare' and 'Your Best Friend' serve as Flowey's themes in Undertale.
In Deltarune however, 'Your Best Nightmare' appears to have been repurposed to represent general fear and terror, particularly in the face of an insurmountable threat.

Additionally, there exists a percussion beat (referred to here as 'Burn in Despair!'), that is re-used in a similar way for Giga Queen's Giga Size :

('Burn in Despair!', as heard underneath 'Your Best Nightmare' in Burn in Despair! )