This is the zone where Valdoartus Varsoon perfected the craft of Planar Thaumaturgy, as evidenced by the inactive portals lining the walls, and discovered his secret to immortality. As you enter the instance, proceed down the stairs and you will see Varsoon the Undying, standing motionless in the center of the room, staring at you.
The zone is a ring event: once you walk close to Varsoon, an event is triggered. He will begin talking to you. After this happens you have a few seconds to get ready before an encounter of 4 void beasts spawn. They do not auto attack, so if you and your group have reached the side room (to the left from the bottom of the stairs) you will have a better chance to prepare.
There are 3 rounds of 4 void beasts, followed by one demoness, an ethereal fiend, after which Varsoon the Undying becomes attackable. It is best to pull the beasts into the side room, so that once they are killed, and the next round spawns, you will have a break to recover health and power.
Both the demoness preceding Varsoon and Varsoon himself are guaranteed to drop a metal chest. If you want to get both chests the max level for the party must be 44 or lower.