The ancient assassin's body was missing once I reached the throne room area in Hag's End. The miscellaneous objective cleared once I pulled the handle to open the passage. This is the first time I've ever had an issue with this quest.
The console indicates that the reference is disabled (or deleted?). Anyway, there was a [D] next to the ID. The SQV output indicates that the quest is running (and the corpse is still held in an alias) until I pulled the handle.
It should probably be noted that I had been to the dungeon at least twice previously on this save, but that shouldn't cause the corpse to never appear.