Archmage Lan'dalock in Dalaran wants you to return with Keristrasza's Broken Heart. This quest may only be completed on Heroic difficulty.
Are you familiar with the story of Keristrasza?
How she was instrumental in the slaying of Saragosa, the mate of Malygos? How, in turn, he took Keristrasza as his consort, freezing her within a block of ice?
We have become aware that Malygos has been using his magic to break her will. This has driven her insane.
[player], you must go into the Nexus in Coldarra and put an end to her before she can be fully turned.
Though it saddens me, I must insist that you bring me her heart as proof.