Abyssal Lich quest bug

I killed the boss in the final stage but died at exactly the same time that he did. When I respawned he was dead and no loot. Re-entering with a new item just brings me to the room with him dead still. I am therefore shut out of the item.
https://imgur.com/7EfNhUR <- death screen
https://imgur.com/4xYt0GU <- what is left after returning

After resetting the area much later with a new item you CAN re-do the event.. but of course I died at the same time again and bugged it again and I don't feel like buying any more invitations.