After you get Mithaniel Marr down to a little less than 50% of his health, he will turn into an unattackable NPC, drop A Lovely Treasure Trove chest and slowly walk towards his sisters' coffin. After a lengthy monologue he will shatter the coffin, update your quest (if you're on it) and earn you the achievement "Love's Avenger."
If you are looking for specific item drops, be sure the instance level is at least the level of the desired drop (e.g., level 80 for Valorous Wings).