When the mad lover of the dread necromancer Kiras'thel was cast aside, the minions of undeath blocked her return to their master. Not one to be stymied, the girl drew forth her violin and began to play. Masterful chords and tones swept over the summoned army and all were rooted in place, the music so powerful and touching that it reached past the boundaries of life and death to shatter the bonds of their necromantic control. The lover danced through the sea of rotting corpses, leaving a trail of entranced undead in her wake. Upon reaching the stunned Kiras'thel, she kissed him, then slit his throat. The army crumbled to ash, and the girl soon joined her lover in death, leaving behind naught but her violin.
A masterwork violin, with nigh-perfect curves, beautiful coloring, and a curled etching along the side of the body: "Happiness is Love"
The magic of the violin is such that it will break necromantic control over any undead within range of music played on it. Most undead are merely entranced, still connected to the necromancer that raised them but unresponsive to commands. The necromancer must reassert control over the affected minions through physical touch.
Undead with more base intelligence (liches, banchees, etc.), however, are not entranced but instead are freed entirely from necromantic control. Such undead are likely to feel more than a small amount of desire for revenge, and even a necromancer rightly fears the wrath of higher undead.
Of course, the music of the violin doesn't just affect undead; the violin also has an affect on the user, albeit a subtle one. As it is played the madness of the original owner seeps into the violinist, infecting them with a growing sense of possessive mania towards one of their companions. The more the violin is played, the more the player is infected. If left undealt with, the violinist will eventually go on a killing-spree, murdering both the beloved companion as well as themself.
Hidden in the Depths
Leaving dangerous artifacts lying around where they can cause trouble is exactly what the Council of Five was created to prevent. After a string of murders surrounding a mysterious violin, the Council took notice and confiscated the violin. Now it sits in their secure vault, though since it's true abilities were impossible for them to divine, it resides in a low-security section.
Unknown to the World
In a musician's vast collection, a masterwork violin rests on a stand with no placard. It has nothing to really justify attention, but it still draws the eye. The musician was took possession of it after the previous owner viciously murdered both her best friend and her own lover in a fit of jealous rage. He has no idea where it came from, and neither does anyone else he's shown it to.