Zereth Mortis was created by the First Ones as a workshop to create afterlives. Its denizens—the automa—have built all that exists in the Shadowlands. It is a strange, otherworldly location that defies all concepts of reality or physics. The Jailer intends to use the secrets of Zereth Mortis to reshape reality and bend it to his malevolent will.
Completing various activities throughout the zone, such as map exploration, collecting hidden treasures, and finishing quest storylines will earn you an achievement, Unlocking the Secrets, that allows you to ride flying mounts in Zereth Mortis.