Zone into the Crypt of T'Haen: Endless Twilight and follow your way till you get to the top where T'rath the Usurper is located. Do not get on the section where he is or near him until you destroy his coffin. His coffin is located at 94.88, 56.72, -65.18 . There is also an alternate exit back to The Tombs of Night on the Highest level of The Crypt of T'Haen at 121.14, 60.16, -105.41 , right past the room with all the coffins on the floor. Destroy the big nice looking coffin to prevent T'rath the Usurper from Complete Healing his Power and Health. If you do not destroy his coffin he will do an AOE lifetap and become almost impossible to kill.
Once dead, return to Nayli R'Ger for your choice of reward and to open up the next quest.
The time has come to end T'Rath. Nayli has determined the appropriate course of action, and even though this will leave another vampire in power in the crypt below, the betrayal the Brethren felt will be avenged.
Nayli has commanded you to return to her bearing the head of the betrayer T'Rath.
Slay the betrayer T'Rath and take his head!
You have slain the betrayer T'Rath.
You have slain T'Rath and taken his head!
Return to the camp near the Timber Falls and speak with Nayli R'Ger.
Speak to Nayli R'Ger in the Brethren of Night camp.
You have spoken to Nayli R'Ger.
You have given the head of T'Rath to Nayli R'Ger.
T'Haen is free and T'Rath is dead. The "disease" shall spread no longer with T'Haen once more in control of his brood. Though the reasons for this are murky under even the brightest light, it would appear that T'Haen has either no desire to muddle with the outside world, or some sort of vested interest in staying away from it completely.