You will find it inside the Ruins of Varsoon at: ( 235, -4, 98 ) /waypoint 235.15, -3.92, 98.24. The access quest is no longer required, but may be done anyway for experience.
The Society of Varsoon Research Notes contain information on the weaknesses of the mobs in the Chamber of Immortality.
This is a zone where the ghost of Varsoon resides. When you enter the zone and go down the stairs, you will see Varsoon the Undying Standing in the middle of a large room.
The zone is a ring event, once you walk close to Varsoon, this triggers the start of the event. 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) 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.