Venture into Tempest Keep's Botanica and retrieve [The Keystone] from Commander Sarannis. Bring it to Archmage Vargoth at The Violet Tower.
This quest is the last step in a quest chain in Netherstorm that begins with  Finding the Keymaster. Commander Sarannis drops the quest item in normal and heroic modes.
It doesn't surprise me that Kael'thas has put the key to this ward in the hands of someone like Commander Sarannis. Prince Kael'thas is a great many things, but he is no simpleton.
In order to break this curse once and for all, you must enter Tempest Keep and claim [The Keystone].
Only once [The Keystone] has been destroyed will I regain my freedom.
Did you obtain the keystone?
<The archmage's eyes light up when you hand him the stone.>
Amazing! From what little I've see of the fate of Farahlon, I'm not certain I want to go outside, but it's much better than being trapped here for the rest of my days.
You will receive: 13 20
A follow up of  Ar'kelos the Guardian.