This page explains how to complete the level 84 Endwalker quest "Heart of the Matter".
This quest is available if you have completed the quest Setting Things Straight.
Starting the Quest
Talk to Growingway in Mare Lamentorum to begin the quest.
Speak with Growingway
Go southwest to the large doors and go west through them to the central area. Then go northwest to the teleporter (the stairs icon on the map) and use it to go to the upper level. Go clockwise around the area to reach the quest icon on the map. Talk to Growingway to proceed. There will be a cutscene.
Defeat the Loporrits
Defeat the three Loporrits that attack, then defeat the next three that attack. Then defeat Growingway.
Speak with Y'shtola
Go southwest along the walkway until you reach the southwest edge of the area, then go counterclockwise until you can go south through the large door. Go to the quest icon and talk to Y'shtola to proceed. There will be a cutscene.
Speak with Urianger
Talk to Urianger to complete the quest.
Unlock Flying in Mare Lamentorum
"Heart of the Matter" is the final quest for unlocking Aether Currents in Mare Lamentorum. If you have completed the other Aether Current quests and attuned to the Aether Currents in the field in this area (which you can find using the Aether Compass in Main Menu > Duty > Collection in this area) you will be able to fly in Mare Lamentorum.
Next Main Scenario Quest
After you complete the quest, talk to Y'shtola to accept the next quest: Returning Home.