Azshara

Everyone who's spent much attention to Warcraft lore knows about Queen Azshara. She was said to be the most drop dead gorgeous night elf ever to have existed and also one of the most powerful magic users during the time before the War of Ancients. She ended up becoming corrupted and tried to open a portal to let Sargeras into Azeroth, who would burn it to a cinder. In the end she was defeated and sent to the ocean depths along with old Kalimdor, right into the clutches of the Old Gods.

Boom. There is born the naga race with the now transformed Azshara as their queen. At this point she is set to become a major villain not in this expansion, who is going to be featuring Deathwing as it's major villain, but possibly in the next one, Cataclysm II, or something (level 85 to 90).

At first I was kind of offended by the idea that one of the most badass villains in Warcraft lore was going to be outdone by a prissy nelf queen turned into a snake medusa thing.

Then I realized a couple things.

1. Azshara has been a villain in warcraft lore for just as long as Netharion/Deathwing has. Remember that his insanity and evil began at the time of the War of the Ancients, which was caused by Azshara in all actualities.

2. Azshara has been sitting at the bottom of the sea for the past ten thousand years as a servant of the Old Gods, and is now the primary matron of the entire naga race. Where Deathwing is all about rage and madness, Azshara is more calculating and manipulative, making very dangerous. The fact is that we don't have a clue really how powerful she's become over the years. We can take a look at Deathwing though and see what the increased influence of Old Gods on Azeroth has done to beaf up his powers. Imagine what they could potentially do with Azshara.