Before the night elves' grand civilization covered the land of Kalimdor, before the troll empire fought the aqir, before the arrival of the titans, they ruled Azeroth. Throughout the cosmos and even on alien worlds, their presence can be felt. Implacable, corruptive, powerful, creators of madness and strife, even in defeat they may never truly be defeated. They exist on many worlds and in many forms, and wherever they exist, chaos, insanity and destruction always follow. They are the ultimate manifestations of the cthonic, the primordial forces of madness that underpin sane, static reality.
They are the Old Gods. And nothing can stop them.
Please note, there will be spoilers for Cataclysm in this post.
What we do know about the Old Gods, we know from their demented followers and minions, really. It's rare that we get much of a chance to talk to an Old God directly, and even when we do, they have a tendency to rant, rave and drive others insane rather than to have much in the way of a discussion. Part of this can be directly attributed to the corrupting nature of these entities, which revels in chaos and tainting and warping the creations and natures of others. In addition, generally the Old Gods (at least the two that have been directly encountered in game to date, C'thun and Yogg-Saron) have a tendency to engage in cryptic or pseudo-prophetic speeches when encountering those minds strong enough to endure their presence for any period of time.
All Old Gods seems to share the following traits: a tendency to warp, twist, deform and drive mad all living things that spend enough time in their proximity. C'thun did this directly in the case of the proto-silithid that became the aqir and later the qiraj, while Yogg-Saron seems to have done so in the making of the Faceless Ones. A third Old God is revealed in Cataclysm, and this entity likewise seems to make use of twisted, corrupted followers. All known Old Gods of Azeroth seem to have the power to either directly or indirectly control or manipulate elementals even to the point of being able to control the most powerful elementals, the Elemental Lords.
Finally, the Old Gods are the only entities in creation known to have ever gone toe to toe with the titans and, if not outright win, then not exactly lose, either. It's true that the Pantheon was sufficiently powerful to imprison three Old Gods within the crust of Azeroth and reorder the roiling elemental chaos these unfathomable alien minds preferred into the world of oceans, continents, rivers and forests that the mortal races inherited. It's also true that C'thun fought a titan to a standstill, and that titan's name is lost to us today, as both Old God and titan fell in that battle. (We do not know if that titan did die, or even if titans can die.) Today, while locked in their ancient prisons, the Old Gods reach forth tendrils of influence and corrupt races, lands, even one of the dragon aspects created by the titans and vouchsafed with watching over Azeroth.
The Old Gods are unfathomable to mortal minds. They seem to delight in chaos, anarchy and constant elemental warfare, pitting the enormously powerful elemental lords against one another seemingly as a form of amusement. The Burning Legion under Sargeras has a twisted ethos that maintains that all existence is a mistake and that all things should be reduced back to entropic nothingness; in contrast, the Old Gods have no ethos, no morality and operate through a raw amorality (one might even say they transcend things like motives and ethics) that seeks nothing more than endless, meaningless, motiveless destruction for its own sake. Under the Old Gods rule, nothing like a society or civilization could ever arise. It's telling that even their former servitors, the Elemental Lords, do not seek their return or serve them willingly, rather seeming to be compelled through powerful sorceries such as the Cipher of Damnation. When even Ragnaros fears and hates something, you know it's bad news. Today, the Old Gods on Azeroth often operate through the Twilight's Hammer cult, who seek the ultimate destruction of the world.
Yogg-Saron was likewise trapped beneath Ulduar in the distant north of the original proto-continent of Kalimdor long before it was shattered into three large pieces during the ancient Sundering. From this location Yogg-Saron sent out its corrupting influence, which can be tracked from the Howling Fjord and Grizzly Hills to the very Icecrown Glacier itself in the form of saronite. Even the very walls of Icecrown Citadel are, in a sense, made out of Yogg-Saron.
Next week we begin looking at each individual Old God known to us in more detail, starting with the first one seen in World of Warcraft, the malevolent C'thun.