Go to Emerald Dragon Island through the Dimensional Gate in Atlantis, get rid of 50 Island Treants, and find Riester.

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Introduction

Atlantis has recently become enveloped by a powerful oppressive energy. Not even Muto knows what is causing it, but believes the source to be somewhere on Emerald Dragon Island.

NPC Dialog

You can feel it too, can you not? This overwhelming negative energy covering the whole city. This sort of energy can only be created by a massive accumulation of grief. I cannot yet determine the precise source of this energy, but it seems to be coming from Emerald Dragon Island.
You can get there via the Dimensional Gate. Go and investigate before this becomes a serious threat to Atlantis.

Quest Completion

(Riester gestures at you. He seems to be asking if you've come looking for the Emerald Dragon, and is surprised that the negative energy has reached as far as Atlantis.)