Zereth Mortis is a new zone added in patch 9.2 “Eternity’s End”. In order to learn how to fly in Zereth Mortis on your flying mounts you will have to decipher the First Ones code and get “Cyphers of the First Ones” achievement. We are offering our help with how to unlock flying in zereth mortis, saving tons of your time and efforts.
ETA for the boost is around 2 weeks.
Zereth Mortis Flying unlocking service rewards: