In the Plane of Hate: Once upon a time, Maestro came with an an AE lifetap and hit for upwards of 200. After the Plane of Hate received a makeover, Maestro is significantly harder.
He hits for a max ~1,300 and spawns a few friends during the fight:
In the Tower of Frozen Shadow: Part of the second encounter within the Tower of Frozen Shadow anniversary raid (see this quest entry for raid information). Upon his death (and Zi-Thuuli's death), a chest spawns with loot.