I could not for the life of me figure out where I was in this quest chain. I had completed all of the prerequisites and Aethas was standing in my Order Hall, but there was nothing to point me anywhere. I checked the Broken Isles, looked for quests in my Order Hall, nada. Finally, after scratching my head for a few days, I downloaded Quest Completist on Curse, used it to query the server and find out which quests I had done, and went through the quest chain one by one. What I found was that for some silly reason (who knows why, really, it was months and months ago) I had not picked up Retaliation! on my mage. I flew out to Suramar, found the NPC, grabbed the quests and was good to go.
So, if you end up stuck like I did, check to see if you've picked up Retaliation!
Second, if that's not it and you have done everything required to start the chain, go grab Quest Completist. It's worth the download.