Locate Apex and obtain Apex's Crystal Focus from him. Return to the Violet Tower and use Apex's Crystal Focus near Archmage Vargoth's Orb to discover who holds the key to breaking the tower's curse.
In retrospect, I suppose I should've expected Kael'thas's magic to be this robust. He's always been a talented mage, but I've never seen anything like the power he wielded during the attack on the tower.
The ward keeps me from being able to scry out the force holding it in place, but it doesn't preclude you from scrying.
You'll require a focus, though. A crystal-bearing giant named Apex roams the plains to the west of Manaforge Duro. One of its crystals would make an excellent scrying focus.
Also, you get: 4 10
Commander Sarannis, you say? Hmm...
She's one of Kael'thas's advisors, but certainly not a mage. I remember her from the attack on Kirin'Var Village. I think your impressions are correct, <name>. She's not maintaining the curse personally, but she holds the key to it.
Upon completion of this quest you will gain: