The day after you quell the final noble's frenzy, a loud sound awakens you.
Go Outside
After you complete The Slumbering Lord of the Tundra and watch the cutscene, go outside.
Afterward, follow Cyllene to the gate.
Seek Out Lian
Go to Grandtree Arena and approach Lian.
Seek Out Mai
Next, go to Heights Camp and fly from there to Worn Bridge, and approach Mai.
Talk to the Person Who Dwells at the Retreat
After the cutscene, cross the wooden bridge and approach the person at the table.
Go Outside Again
Check on the door to go outside.
Choose Whose Help You Want
Speak to either Irida or Adaman to help you. You won't be able to change this choice later.
Approach your chosen guide to exit the Ancient Retreat. You can do the lake trials in any order, so choose whichever location you prefer.