This page explains how to complete the level 80 post-Shadowbringers quest "Unto the Breach".
This quest is available if you have completed the quest Futures Rewritten.
Starting the Quest
Talk to Alisaie in the Rising Stones in Mor Dhona to begin the quest. There will be a cutscene.
Speak with Urianger in the Ala Mhigan Quarter
Teleport to the Ala Mhigan Quarter in the Lochs. Go northeast to where the orange arrow is pointing in the minimap. Talk to Urianger to proceed. There will be a cutscene.
Speak with Arenvald
Talk to Arenvald to complete the quest. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, talk to Alphinaud to accept the next quest: Here Be Dragons.