Enter the Violet Tower at the center of Kirin'Var Village and defeat Ar'kelos, then speak with Archmage Vargoth.
Now that you've gathered all the items I'll need to break Kael'thas's curse, one thing remains to be done. When Kael'thas sealed the tower, he placed an arcane golem, known as Ar'kelos, at the bottom of the tower.
You must defeat Ar'kelos in order allow me to work magic within the tower again.
When you enter the Violet Tower, the curse will begin to affect you.
Once you have defeated Ar'kelos, ascend the tower and assist me in making preparations for the ritual.