All info from Wowhead
Flying in Zereth Mortis requires completion of the achievement ‘ Unlocking the Secrets’
You must complete the following achievements to earn this meta achievement:
Explore Zereth Mortis: Explore Zereth Mortis, revealing the covered areas of the world map.
Curious Collections: Discover 5 hidden treasures of Zereth Mortis.
Path to Enlightenment: Complete 3 side questlines in Zereth Mortis.
Tales of the Exile: Read the volumes of “Firim in Exile” found in Zereth Mortis.
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Adventures in Zereth Mortis: Complete 10 of the following special encounters in Zereth Mortis.
A Means to an End: Complete the storyline “A Means to an End”, or Chapter 6 of the 9.2 Campaign.
Once this achievement is earned at least once, Zereth Mortis flying is unlocked for your entire account.
None the Wiser Guild 