I went back to oasis and turned the quest in.
I also activated the new ones (incl. the Leviathan/Treasure Room one by accident) but i don't think that matters to make him disappear.
Just try to turn in the quest then go back.
Here is the video wich i found most helpfull:
Hmm...why doesn't embedding work anymore, or am i doing something wrong?
Usualy i just had to copy&paste the YT link from the adress bar to have an embedded player, iirc...