Use the Dimensional Gate to reach Emerald Dragon Island, then speak to Riester.

Show Quest Info

Introduction

Muto has suggested that you continue on to Emerald Dragon Island to seek the next of the Four Guardians: the Emerald Dragon. His warning sits at the back of your mind; there is no telling what you can expect as you proceed. Make your way to Emerald Dragon Island through the Dimensional Gate and look for the Emerald Dragon.

NPC Dialog

Remember what Delai told you, young warrior. We
don't yet fully understand the true purpose of the
Four Guardians, so be careful. You can reach
Emerald Dragon Island through the Dimensional
Gate. It's a distant place that very few have ever
ventured to, so there's no telling what strange
creatures you may encounter. Go there and seek
out the Emerald Dragon.

Quest Completion

(It doesn't seem he's able to speak in any human
tongue, but his gestures seem to be telling you that
this place is dangerous and you should leave.)