Earth Mastery. One of the 8 available Masteries you can choose from.
Earth Elemental Mastery allows the ninja to learn Earth techniques that have large area of effect and effectively control crowds.
Earth Mastery offers great crowd control capabilities with multiple stuns, the biggest being Earth Prison which when mastered will stun everyone within an 8 tile radius for 3 seconds. It also has a homing, a knockback and a piericing with range. A drawback is the cast times making it somewhat difficult to use some techniques in the heat of battle.
For PvE it starts off slow since the first two techniques you learn are 1 single target techniques, but it does get easier once you learn more techniques once you get access to the long range piercings.
For PvP in 1v1 battles it's quite hard to use some techniques since you have the cast times but this mastery greatly shines in groups. If no one is aware of you, your Earth Prison will trap a lot of enemies, allowing you and your allies to defeat your foes. It also has a long range piercing stun with Ravaging Earth Spikes allowing you to hit multiple targets and stun.