No matter how many times I look I can't find it, and I don't want to completely redo the dungeon. Help?
You'll find the oyster against the outer wall of the inner ring that surrounds the Tree of Life, where the Wraiths roam. Just follow the whole wall, and you'll stumble across it. It blends in quite well with it's surroundsing.
It's in the "little courtyard" with the tree in the middle. It is very hard to see. Walk around the little courtyard that surrounds that tree. It's in there.
The tree of life area has 3 rings of walls. It is found against the inside of the 2nd ring.
If you go to the tree itself, there are 2 entrances. One is close to the guy who gives you your quest. Go out the other entrance and walk straight to the 2nd ring and you will find it.
No matter how many times I look I can't find it, and I don't want to completely redo the dungeon. Help?
Once you enter, go to the tree of life. In the building with the tree of life, go to the inner circle. On the side opposite the quest-giver/where the portal appears is where you should start your search.
You don't need to repeat the quest to find that one. Stay inside the walls of the fort around the tree. I don't remember if it's in the inner part or the outer part, but you should find it there.