Essence of Aszune

Legends speak of an intelligent orc who found the Heart of Aszune. This is the essence of her heart, precious to the night elves. It has the power to heal the hero who wields it. This item is permanent.

Source
Kael and Lady Vashj can find this in The Dungeons of Dalaran, chapter 3 of the Frozen Throne alliance campaign, Curse of the Blood Elves.