Bloodline Chronicles adventure pack required, but only one member of the raid needs to have gotten access for the raid zone, this same person can zone the entire raid into Tombs of Night, and then into the Vengeance instance.
Zone in and run around the bottom of the water area clearing the mobs that are spawned next to the glowing and clickable orbs, each orb area will spawn a named. Then run up the pillar to the hound area on the second floor and kill another named and click the final orb.
With the orbs turned off, the wall of magic opens and you can run to the top to kill T'Haen.
Once up top, kill the trash to clear the area. When you get close to the boss, T'Rath the Usurper will die instantly and T'Haen will become attackable, although some bats will spawn on top of you that need to be taken care of first.
This boss is a DPS check, you must kill him as quickly as possible and ideally not long after he begins creating copies; he also has a minor AE knockback that throws you about two feet at the most, so be sure you're not standing at the edge of the platform.. Just have the tanks grab the copies and hold aggro but do not kill them. All DPS should keep working on the original T'Haen the entire time. If you do not kill the boss in 45 seconds or so he will begin to spawn copies of himself. It starts with three copies and then multiplies fast. At 95 seconds you can have the boss and as many as fifteen copies attacking you (which would imply a set of three copies created every 10 seconds).