Land of Turquoise
This mission begins automatically when you arrive in Sinai.
Use Senu to identify the site of the local Hidden Ones Bureau, and make your way there.
The entrance is, of course, hidden. Use Senu again to identify a secret passage in the rocky bluff behind the bureau building.
You don't need to read the scrolls in the first room, or talk to the trainees in the second room (at least, not yet). Instead, follow your marker to Tahira for some video exposition.
The end of this cutscene completes the quest.