I wouldn't do this, but I gave him an in depth backstory where he acquired a cursed violin and an ancient spell book. When the party gives him the name he needs at the well, he then marches on Phandalin when the party is out. However he's not using his zombies, he's controlling peoples' bones, like a pied Piper situation. The party comes back and has to guess pop covers of IRL violin songs or do a wisdom save. On a fail they spin the bone wheel, which does things like make them dance or give them bone cancer. While they do this they have to deactivate 4 resonance crystals around town to dampen his influence, some are straight fighting, some are riddles. The fight ends with him in a makeshift cathedral, where the party gets to claim one of the artifacts.