At the Thyphlo Ruins, one of the stone slabs says to demonstrate the power of the Sage of Water to the six dragons in the area.
How to Complete the Quest
Before you can complete this quest, you have to complete the Water Temple. Then go into your key items and summon Sidon. The six dragons are in the northwest part of the island, and they are looking at the location at 0186, 3197, 0174. Stand there and use Sidon's ability, and when you are in the water bubble, swing your weapon.
Quest Reward
The chest contains 5 Opals.