Once you get the key, return to Evermore Greens North. The gate that was once closed is now open. Enter the tunnel that is where the gate used to be. Once in the cavern, follow the maze until the end. At the end, you should find the Giant Crab. The Giant Crab is level 30 and has 4700 HP. It can take a while to defeat him at lower levels. Once defeated, you will get exp, 2,500cR, the Water Crest , and an Agility Shard. Then return to Neem town to redeem the rewards for the quest which include a Water Dagger, 20,000cR, and other stuff.