This is a mostly OOC thing. I cannot link the site page, due to some spoilers about AQ it presents *dammit, god bloody dammit a good surprise ruined* However, here is a small excerpt of text...

There are only 5 Old Gods in all of Azeroth. 3 were chained deep beneath the surface of the world, one was killed, and one is unaccounted for. The 3 continued to exert their will, though in far more limited ways.

-They corrupted the great benevolent Dragon Aspect Neltharion, who created the extremely powerful Demon Soul, which they were to use to set themselves free. The Demon Soul was taken from Neltharion and eventually destroyed.
-They corrupted the Highborn of the Night Elves, and used them to lure the Burning Legion to Azeroth. Many of the Highborn were transformed into the Naga, who serve the Old Gods still.
-They managed to throw much of Azeroth into chaos with the destruction of the Well of Eternity and the weakening of the guardian Dragonflights, including driving one of the 5 Dragon Aspects mad.


Aside from this, it is also found in the War of the Ancients Trilogy that the Old Gods were playing the legion themselves, not directly, but using them as a method to regain entry into Azeroth, where they would omgwtfpwn everything. Malfurion, the dragonflights and Krasus the Dragon-mage at the very least found this out, thus the dragonflights took action, despite Neltharion still wielding the Demon Soul.