To complete this instance and spawn the boss at the end you must do all the ring events. In total you will have 3 towers and 2 sacrificial wells to clear for the hearts and minds it requires to spawn Terror. Place these items in 6 offering bowls at the front of the pyramid to spawn the boss.
Make sure to clear the mobs around all 3 of the towers to spawn the boss for each Each drops a skull with a sword that should be looted by all group members. For a complete walkthrough of the ring events, see the main questline.
This new Dungeon zone was announced at FanFaire 2007.