Imprisoner Y'nosii  

Clear the mobs between the door and Y'nosii, then each player should click on the post before a cell to possess that prisoner and attack Y'nosii! After the battle Y'nosii's weakened soul will spawn. Cancel the possessions one at a time and kill the prisoners, then kill the weakened spirit to get a key to the chest in the room.

Any other sequence will lose the key and the chest!