After bringing Illas' Pendant to Vednidher in Hawksmouth, you have to lure him to the graveyard. Everytime I reached the graveyard with him, he disappeared after turning attackable. I tried it 3 times and varied the routes to the graveyard, it happened every time.

I was able to complete the quest with a hint from Acid. When Vednidher disappears, you can quickly run back to his start position and kill him.

Ok, this was a pain, but I got it sorted. Fixed locally, will push with the next update