WoWHead has this listed under their Patch 5.1 Dominance Offensive Guide as the first quest under the Blood For Blood section of quests. I did not receive this quest first. Instead, I received To Mogujia. I was on a blood elf so I'm not sure if we get a different quest or if they have just changed the quest text/title since WoWHead made their guide. I think my rep before turning in the quest I mentioned was about 3245 Honored if that helps anyone wondering when this will open for them.
EDIT:
So since writing this comment up, I have now received this quest at about 7500 Honored. This is the sealed communication you receive:
<Name>,
First, I would like to apologize for the cryptic note - but discretion is imperative.
As past events have taught us, others in the Horde may act without proper planning, or restraint.
We plan to move first towards our next objective and avoid further pointless losses.
You have shown yourself to be a highly skilled and reliable ally. We would like to invite you to join us - as soon as possible - at the Valley of the Emperors in Kun-Lai Summit.
We hope you will meet us with all due haste.
-A Friend from Silvermoon
After turning in this quest, I recieved Legacy of the Korune and Clearing a Path. Once I turned those two in, I received Trapping the Leader. (Note: This quest is listed under The Korune achievement section in the Dominance Offensive Guide.)