Retrieve the Sigil of Krasus from Spellbinder Maryana at Manaforge Coruu, northwest of the Violet Tower. When you have gained the sigil, use Archmage Vargoth's Staff to project an Image of Archmage Vargoth and inform him of your progress. If you lose Archmage Vargoth's Staff, speak to Ravandwyr at Area 52 to receive a replacement.
Before we departed Azeroth all those years ago, the Kirin Tor entrusted our expedition with a powerful artifact, the Sigil of Krasus. It was a great boon to us in our battle against the orcs and it could play a key role in helping me break the enchantment that imprisons me.
However, when Kael'thas divested me of my possessions, he gave the sigil to Spellbinder Maryana, who remained behind with the garrison. If he truly realized what he had in his possession, he wouldn't have given it away so easily.