According to Blizzard:
Plagueworks has just opened today, starring Professor Putricide and his buddies, Festergut and Rotface. And before you have time to yawn at the lack of new challenging content, the next wing will be open.The Crimson Hall contains the leaders of the San'layn, undead blood elves who oversee the Scourge's operations throughout Azeroth. The blood-princes Valanar, Keleseth, and Taldaram were raised into undeath by the Lich King to avenge themselves while protecting their blood-queen, Lana'thel.
We'll keep you updated on any details, as usual.