The Sunken One glitch - Slythe's body not appearing

[Valerie]

I had to look it up and found that this is a common glitch, but I haven't found any way of solving it.

It may have glitched due to the fact I did this dungeon BEFORE getting the quest, but even after I got the quest it seemed to respawn all the enemies and coming back didn't spawn the body to end the quest.