Defection from Kael'thas
Formerly a magister under the command of Kael'thas Sunstrider, servitor of Illidan, Voren'thal led an army of blood elves against the Sha'tar and their Aldor allies who had reclaimed the shattered Shattrath City. But as the Aldor vindicators and exarchs prepared to defend their city, the unexpected happened - the blood elves put down their weapons before the draenei defenders. Voren'thal stepped forward and demanded an audience with the master of the Sha'tar, the naaru A'dal.
As A'dal approached him, Voren'thal went to his knees before the naaru and said, "I've seen you in a vision, naaru. My race's only hope for survival lies with you. My followers and I are here to serve you." A'dal accepted the offer of assistance from Voren'thal and his men, who became known as the Scryers. This defection proved to be devastating to Kael, and subsequently to Illidan; Voren'thal's forces had included many of Kael's most gifted scholars and magisters.
After the defection, Voren'thal and his followers set a base in what is now known as the Scryer's Tier in the southern part of Shattrath. Voren'thal himself continues to lead the Scryers from within the Seer's Library at the center of the Tier, where he directs the Scryers' efforts against both Kael'thas and Illidan.
According to the quest text in Turning Point, Voren'thal is in rather poor health; he says "My frail health prevents me from aiding you in person with this endeavor", leading us to believe he is not well.
Also, according to the quest text in Bound for Glory, he also appears to be old in relation to other blood elves; Spymaster Thalodien says "Make sure Voren'thal doesn't topple over with excitement from the news - the old man's getting a bit fragile, if you know what I mean.", also pointing once again to the fact that he is ill.
Voren'thal is involved in the following quests:
-  (given by Khadgar in Shattrath)
-  (given by Spymaster Thalodien in Netherstorm)