Instructions
Proceed through the beginning part of the Plane of Water by following the linear path until you get to an area with a portal and three caves to the east, west and north.
Enter the cave to the west. Explore the entrance to find a monolith called the "Stone of Coirnav." Push the stone through the cave until you come across some Undead Skeletons and Water Demons. Fight them and continue pushing the stone. Repeat this process until you get to the end of the cave. Near the portal at the end of the cave is a hole -- push the stone into the hole. Use the portal to get transported to the area with the portal and three caves.
Enter the cave to the north. Like the previous cave, you must find the Stone of Coirnav and push it to the end of the cave while fighting off Undead Skeletons. Some players find it easier to leave the Stone of Coirnav at the entrance, explore the cave to clear it of enemies, then go back for the stone -- use whichever strategy works best for you. Push the stone into the hole and use the portal to go back to the beginning area.
Enter the final cave -- the one to the east. Again, you must push the Stone of Coirnav to the end of the cave. However, this cave is not as winding and is more or less a straight shot from beginning to end, making it a bit easier to navigate. Push the stone into the hole at the end of the cave to initiate a cutscene.
Watch the cutscene to see a large clam spit out several items, including the magic shard for the level. Collect the items and the magic shard, then use the portal to teleport back to the Plane of Tranquility.