You seek the beacon that the mysterious voice told you to find at the Crystal Tower.
Requirements
This quest is available if you have purchased the full game (see Getting the Game for more information), and you have registered the Shadowbringers expansion to your account (see Adding an Expansion to the Game for more information), and you have completed the quest A Requiem for Heroes.
Starting the Quest
Talk to Tataru in the Rising Stones in Mor Dhona to begin the quest. There will be a cutscene.
Speak with the Saint Coinach Ferryman
Go outside and go southeast to where the orange arrow is pointing in the minimap. Talk to the Saint Coinach Ferryman and say Yes to go to the survey site.
Speak with Tataru
Talk to Tataru to proceed. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Head to the Crystarium
Go southeast to where the orange arrow is pointing in the minimap. You can't use a mount here, so just go on foot. When you enter the orange circle, there will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Speak with the Crystal Exarch
Talk to the Crystal Exarch to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to the Crystal Exarch to accept the next quest: City of the First. There will be a cutscene.