While we've been busy putting Cho'gall in his place, dealing with Nefarian, and otherwise addressing Elemental Lords Al'Akir and Ragnaros, the rest of Azeroth has been busy in its own way. Players wandering through the Badlands are taken along on quests to help the red dragon Rheastrasza perform a bit of important research on the black dragons. Though at first it seems as though Rhea's work is mundane, her ultimate plans are soon revealed.
Rheastrasza is attempting to find one purified, untainted black dragon egg. And she's successful, but the egg is in immediate danger. Deathwing doesn't want this egg to exist, and he'll do anything to get rid of it. The egg has been shuffled around constantly, hidden from Deathwing's sight -- a faint shred of hope in the post-Cataclysm world.
... unfortunately, nobody bothered asking the egg's opinion on the subject.
Today's Know Your Lore contains spoilers for the rogue legendary quest line.