After the unexpected turn of events, it is up to you to deal with the new threat.
This quest is available if you have completed the quest Breaking and Delivering.
Starting the Quest
Talk to Alisaie on the Isle of Zekki in the Ruby Sea to begin the quest.
Unlock the Vault Door
Go northwest, then north to the orange circle on the map. Check on the sparkling Vault Door to proceed.
The Pool of Tribute
Use the Duty Finder to enter The Pool of Tribute. Please see the The Pool of Tribute Guide for a walkthrough.
After you complete the duty in The Pool of Tribute and exit the area, 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 Lyse
Go northwest to where the orange arrow is pointing in the minimap. Talk to Lyse and there will be a cutscene.
Speak with Lyse Again
Go to the quest icon on the map and talk to Lyse to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Lyse to accept the next quest: Tide Goes in, Imperials Go Out.