If you're not that far into Dragonblight quests and don't want to be spoiled, I'm putting it behind the cut.
The quest is A Letter for Home, and if you're doing or you've done the questlines in Dragonblight concerning the Moonrest Gardens (most of which are identical regardless of your faction), eventually you'll run across a drop from an important leader amongst the Blue Dragonflight forces. The Alliance will get Captain Malin's Letter; the Horde will get Lieutenant Ta'zinni's Letter.
Both letters have been written by people (the former a human, the latter a troll) who were blackmailed into service for the Blue Dragonflight under threat of harm to their families, but are secretly working to destroy the dragons' efforts from within. You kill the turncoats and then you get the letter drop explaining this sad state of affairs, and decide to take it to your local commander to see if there's anything that can be done for this person's family.
Now, Overlord Agmar, the guy who's running the eponymous Agmar's Hammer for the Horde in Dragonblight, is no bowl of chuckles under ordinary circumstances. He's slightly stressed and utterly disgusted by three other traitors running around the area. He is one of the most unsentimental and least forgiving NPC's you will find in the entire game. As far as he is concerned, you are a member of the Horde. You are aware of the problems that the Horde (and, he will grudgingly admit, the Alliance) faces in Northrend; if you weren't, you wouldn't be here. You will do your job and you will do it to the best of your ability, aware of all the things that ride on it, and he will not pity or accept cowardice, hesitation, or -- God help you -- betrayal, because you do not have the luxury of thinking only of yourself.
So I wasn't expecting him to react well, but I also wasn't expecting this:
<The condescension in the overlord's deep-throated laughter is palpable.>
What did I say? Traitors! Still, I knew Deino once upon a time. She at least is a troll with honor!
I'll see to it that she finds out what happened to her brother. I'll even forget to tell her that you're the one that killed him.
<Agmar fixes you with a calculated stare.>
But I own you now, <class>!
A lot of people playing Horde will cite Message From the West as being a sort of epistolary counterpart to this quest, and it's very true that Saurfang's letter washes away a lot of the unpleasantness you've probably dealt with while leveling (as does Saurfang's conversation with Garrosh Hellscream in Warsong Hold; hang around them until you see it). There's a lot of infighting over what truly constitutes the soul of the Horde. I can only hope that Saurfang's concern prevails over Agmar's indifference.