Archmage Lan'dalock in Dalaran wants you to return with the Head of Cyanigosa. This quest may only be completed on Heroic difficulty.
We're being invaded! The blue dragonflight has teleported into the Violet Hold and they're planning on breaking out from there into the rest of Dalaran!
There have already been reports of prisoners inadvertently being freed by their actions. It wouldn't surprise me if they're purposefully letting some of them free.
[player], can you assemble your team quickly and get in there? Cyanigosa is the leader of their invasion force. Bring me her head!