To unlock the quest for the fight, you just have to get the trow the 50 hidden music sheets. He’ll join your party as a guest in search for the final sheet in the onyx theater. At the end of the dungeon he’ll transform into a crazed corrupted fairy and the fight will begin.
He’s pretty easy for a secret boss. Just bring a bard with Active Noise Control to cancel out his stat effects. After the fight he turns back to normal and grants you the bard spell Eventide Advent. Playing it for a character allows them to transform into a monster version of themselves for a major boost in all stats, but a loss of control till a battle’s end.

Happy Orctober!