A Vicious Twist  

After completing the Shattering the Crystals quest (see related quests) Arthial the Protector will open up the "A Vicious Twist" quest. He will now ask you to go kill T'Rath the Usurper, the boss vampire in the Crypt of T'haen: Endless Twilight.

To do this, you must bring a full group with a heavy tank and a couple of healers. T'Rath will bring up some nasty AoEs that blows casters down to almost zero health - around 2k dmg in one hit. A good idea is to use the stun/stifle items you can buy from the new faction merchant and stifle him so he cannot cast.

Before engaging him, make sure you destroy "a magnificent coffin" from where it's said he will get his powers to heal. You will find this coffin off one of the small passages that leads off the pedestal where T'Rath stands 94, 56, -66 Copy .

After it is destroyed, you can now go kill T'Rath.

When T'Rath is down, you can return to Arthial to get your reward.