Take Quel'Delar to the Sunwell as quickly as possible, <name>. Trust in Uther's words when he says that you will not always be able to control the blade while it is subject to the evil of the Lich King's influence. You must travel to the Isle of Quel'Danas, and there, seek entry to the Sunwell itself, where you can cleanse the sword.
The guardians of the Sunwell will not simply allow you to walk in, so you must find Halduron Brightwing, who will be inspecting the guards there.
Admit you to the Sunwell? That's impossible! The Sunwell is the most sacred and important site of our people. We have only recently reclaimed it with the help of the Shattered Sun Offensive. It is not a place for tourists or travelers.