Zaralek Cavern is available for testing in the first Patch 10.1 PTR build! We checked out the entrance to the new zone.
The Entrance to Zaralek Cavern is located by the chasm between Thaldraszus and Ohn’ahran Plains, and the entrance can be seen on the map for both zones with a cave icon.
There is currently no intro questline leading to Zaralek Cavern, so you can just go to that cave and fly into the new zone! You can see a flythrough at the cave in the video below: