looks like the main Boss in this event is the Mini Boss that respawn in 3rd wave, because I looted a Emeral and Thyrst Guard vanity, and I looted also Golden Clover Artifact.
But the Last wave Boss Guttus Thyrst the drops is just Ashen vanity and Locks, never have a good drop..,
to bad 3rd wave mini Boss not respawn everytime..😊