The most prominent feature of the accompaniment in the first movement of Beethoven's Moonlight Sonata is best described as rolling arpeggios.
What is Beethoven's Moonlight Sonata?
Beethoven uses a recurring pattern of arpeggios, or broken chords performed in a rolling motion, in this section. These arpeggios are performed in the pianist's left hand and make up the accompaniment. The movement has a unique and captivating aspect because of the arpeggios' flowing and cascading effect throughout.
The melody line performed in the right hand is supported by the arpeggios in the accompaniment. They give depth and richness to the piece's overall tone and impart a sense of momentum and energy.
Beethoven creates the eerie and hypnotic mood that is recognizable to the Moonlight Sonata by making rolling arpeggios the main theme of the accompaniment.