Clear The Collapsed Sanctum the first time you enter it, including non-aggro elementals. These will add and debuff.
The Earthen Behemoth spawns in the Collapsed Sanctum ( 66, -12, -143 ) Copy/waypoint 66, -12, -143 after killing Unatu and Poxrata.
During the fight, the Earthen Behemoth breaks into smaller dislodged stone adds.
If the dislodged stones aren't killed quickly, the Earthen Behemoth will suck them back up and they will heal him. Their health is low so they can be taken out easily.
The Earthen Behemoth has a knock-back that throws you up in the air.