You rush to Lyhe Ghiah upon receiving word that Ran'jit has arrived in Il Mheg.
This quest is available if you have completed the quest Memento of a Friend.
Starting the Quest
Talk to Seto in Il Mheg to begin the quest.
Investigate the Castle Gate
Go southwest to where the orange arrow is pointing in the minimap, but don't swim into the lake. Instead, go southwest along the water's edge and the cliff top to reach a stone stairway. Go up the stairs and you will reach the quest icon on the map. Check on the sparkling Destination to proceed. There will be a cutscene.
The Dancing Plague
Use the Duty Finder to enter The Dancing Plague. Please see the The Dancing Plague Guide for a walkthrough.
After you complete the duty in the Dancing Plague 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. (Main Scenario 1 > Shadowbringers Main Scenario Quests > Acht-la Ormh Inn > Cutscenes 3 and 4.)
Speak with Urianger
Talk to Urianger to complete the quest.
Unlock Flying in Il Mheg
"Acht-la Ormh Inn" is the final quest for unlocking Aether Currents in Il Mheg. 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 Il Mheg.
Next Main Scenario Quest
After you complete the quest, talk to Thancred to accept the next quest: The Wheel Turns.