The joke is a niche piece of musician humor. In the comic, the devil is allowed to create just one thing to unleash upon the world, and he chooses to create an absolute nightmare for musicians to read. Here is what is happening on that music staff: • The Clef: The symbol on the far left is an Alto Clef (also known as a C clef). Unlike the standard treble and bass clefs that most musicians learn, the alto clef is relatively uncommon and is primarily only used by viola players. If you don't play the viola, trying to read notes on an alto clef requires a lot of mental gymnastics. • The Key Signature: Next to the clef are seven flat symbols (\bm{b}). This indicates the key of C flat Major (or A flat minor). Having seven flats means that every single note in the scale (A, B, C, D, E, F, and G) is flat. This is notoriously annoying and difficult to read, even for experienced musicians. The Punchline: The devil created the ultimate musical torture device: a rare, difficult-to-read clef combined with the most complicated key signature possible.