Alphinaud asks you to return to the Waking Sands to resume the activities of the Scions of the Seventh Dawn.
This quest is available if you have completed the quest Lady of the Vortex.
Starting the Quest
Talk to Alphinaud in the airship landing of Ul'dah to begin the quest. (If you are outside of the airship landing, you can speak with Elyenora in the airship landing area to reach Alphinaud.)
Speak with Alphinaud in the Waking Sands
Use a Vesper Bay Aetheryte Ticket (which you received after completing the quest Back from the Wood) to return to Vesper Bay.
Once in Vesper Bay, go into the northwest building and go down the stairs to enter the Waking Sands. Once inside, go east and talk to Alphinaud to complete the quest. 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.
Next Main Scenario Quest
After you complete the quest, talk to Y'shtola to accept the next quest: Casing the Castrum.