The end of "A Destiny of Flame and Sorrow" after Illidan absorbs the Skull of Gul'dan and kills Tichondrius.
Malfurion: "Foul demon! What have you done with my brother?"
Illidan: "It is I brother. This is what I've become."
Tyrande: "Illidan, how could you?"
Illidan: "The leader of the demons is dead. And the forest will heal, in time."
Malfurion: "At the cost of your soul? You are no brother of mine. Begone from these lands and never step foot here again."
Illidan: "So be it... brother."
The short "Dreadmaul Rock" quest chain in the Burning Steps. A troll asks you to check on his wife Sha'ni who was out on an investigation and hasn't reported back yet. You go to the area in question, and there's no sign of her... until you click on some bones lying on a stone platform. Sha'ni's ghost appears and explains that her platoon was captured by ogres. Everyone but her was executed - she was strapped to an alter by the leader and brutalized. She died an hour into the torture, at which point the ogre ripped off her wedding nose-ring as a trophy. Sha'ni can't rest until her husband knows her fate, so you find and kill the ogre leader and bring Sha'ni's wedding ring back to her husband:
Thal'trak: Did you find Sha'ni? Is she okay?
Thal'trak: We were going to move to the Hinterlands. Did she tell you? This was going to be her last mission...
(You give him the ring. Thal'trak sobs.)
Thal'trak: I don't want it. Keep it, throw it away, I don't care! The ring means nothing to me now. Not without her...