Can't Start Zaralek Caverns Intro Quest "The Land Beneath"

In order to start the patch 10.1 campaign to open the Zaralek Cavern, you first need to complete the Forbidden Reach campaign up to A Creche Divided quest.

Finishing Scar of Earth and Fire quest on one character will unlock tunnels to the Zaralek Cavern account-wide, however, the Forbidden Reach campaign still needs to be completed or skipped on each character separately to get access to the 10.1 campaign.

Note: Hadexian may not be where the quest bang indicates on your mini-map for The Land Beneath, depending on your quest phasing. Check the western side of the platform if you don't see him where he should be.