A violin sonata by the Italian composer Giuseppe Tartini (1692-1770). The piece is often considered one of the most difficult works for that instrument. Looking at my copy of the sheet music, I can understand why. I've played the violin for some seven years (though, having stopped actually learning late in high school, I'm not exactly at the peak of my ability), and I have no hope of being able to get past maybe two pages. Ever.
The violin sheet music for the arrangement I have is eight pages long. It starts slowly, but picks up the pace before the violinist has to turn a page. The music itself, once you pass the introduction, is almost entirely eighth and sixteenth notes -- not that intimidating on the face of it (Pachelbel's Kanon in D had a couple of sections with thirty-secondth notes), but there are two things about this which inspire paralyzing fear.
First, they're played fast. Allegro assai is a common sight in the notes, and at one point Tartini apparently invents his own tempo -- not knowing Italian, that's my guess as to what "Tempo giusto della Scuola Tartinista" means.
Second, Tartini seems to have had eight fingers on his left hand. That's the only explanation I can give for his insane chords, reaching across multiple strings, thrown one after the other in blistering succession. Near the end of the piece, there are sections in which the violinist is supposed to play trilling chords -- two fingers alternate notes as fast as possible on one string, one finger holds down another string, all while the right arm scrapes the bow across.
For all that it's next to impossible to play, the Devil's Trill is an astonishingly beautiful piece of music with an interesting history. According to legend, Tartini saw Satan himself in a dream, standing at the foot of his bed. Not having anything else to do, the composer handed his violin to the devil, who promptly played the "original" Devil's Trill. Tartini was so captivated by the melody that he stopped breathing; supposedly, the shock woke him. He instantly grabbed the violin and tried to reproduce the devil's tune, but was never satisfied with his rendering. The sonata was only published well after his death.