This applies to "an Silver Ancient Wurm" that spawns on the east platform. You must kill him before he roams past either the north or south pillars at the bottom of the north & south stairs. He stands still for roughly 30 seconds, then he will begin to roam. You can use movement pulling abilities on him such as paladin lurch and monk fist. Keep pushing him back if you don't have the dps to kill him before he roams. If he makes it to the pillars, his lightning AE goes out and you've lost it.