At the Thyphlo Ruins, one of the stone slabs refers to a corridor between two dragons.

How to Complete the Quest

Before you can complete this quest, you have to complete the Fire Temple. Then go into your key items and summon Yunobo. The corridor is on the west side of the island, just east of the bridge, and the dragon's heads are facing the location at 0204, 3123, 0174. Stand there and use Yunobo's ability and have him go through the corridor between the two dragons. This will reveal a chest.

Quest Reward

The chest contains 3 Rubies.