He visited the Forbidden Forest for a reason and that was to find unicorn hair. His adventure with Hilary was a great coincidence which proved fortuitous.
Opening the case, a sleek mahogany violin was perched carefully in it---well used, but still beautiful. But he reached not for it, but for the bow and disentangled the common horse hair from it.
After doing so, he pulled out the bushel of unicorn hair acquired from his trip. Silver from the adults and gold from the foal that was witnessed while visiting. Making sure it was neat and tightly wound, he attached it to his bow.
It is said that playing music with unicorn hair would yield almost the most magical of melodies. And that was something he'd happily try out to see if the rumors were true.
The bow now tightened with the hair, he finally brought out the violin and tuned it, the soft plucking of musical notes invading the quiet air. When he was satisfied with the tuning, he finally stood up in position, the bow resting along the violin's strings.
He gave himself a moment to breathe before immediately setting to it, driving the bow into a quick succession of Bach's Concerto in D Minor. While it was made for two violins, the unicorn hair seemed to make up for the other violin, providing two sounds that reverberated throughout the walls of the tower.
The music was completely and irrationally beautiful, even from his six years of violin practice he had not yield such sounds as he was now. The violin seemed to have a voice of it's own and began to sing with a bountiful air. It livened up his spirits.
Will continued to play, for the music was so beautiful he could not bear to quiet it for a second. It was pure magic thanks to the unicorn hair. Magic was given a voice---a sound.
He could play forever.