In Act II there is a side quest called Descent into Torment offered by Kalderos to kill the Grand Priest Zarthuzellan. Once you kill him Kalderos will appear and after speaking to him you'll receive a skeleton key. Use the key to open the doors behind Kalderos and enter the 4th floor of the Steps of Torment called Suffering.
The Chamber of Souls is a room located relatively close to where you enter this floor. In the center of the room is a large chest surrounded by green mist. When you enter the room a gate will close behind you and you need to survive 3 waves of enemies. If you survive the waves of enemies be sure to click on the chest to unlock the achievement.
Note: Portals do not work on this floor and if you die you'll need another skeleton key to re-enter the floor.