Moonlight Sonata is a composition by Ludwig van Beethoven for the piano that also serves to be of significance to Malice Manifestation.
It is also used as the third key for solving the horrors that lie within.
The clue for this key can be found in the sheet music:
Each of the movements are used for their purpose:
Psst! It's me, A-120! You gotta start running when you hear this song if there's no piano nearby!