The Old Gods are legendary parts of WoW. You've got C'Thun hanging out in southern Azeroth, whose unblinking eye stares balefully across the world. Those of us who were part of the Northrend expedition certainly encountered Yogg-Saron. Heck, many of us got so familiar with Yogg-Saron that we made armor out of his blood and danced around Icecrown Citadel in our favorite made-from-death-god prom outfits.
The Old Gods are a defining part of the World of Warcraft experience. They are unmistakeably influenced by the Lovecraft mythos, wherein ancient gods from the stars balefully disregard the lives of humans. (Remind you of a certain heavenly-bodied heroic boss fight?) An equally important part of the mythos, however, is the unknowable, abyssal fathoms of the ocean's deeps. This is a world where humans can not tread, where light does not shine, and where we can not know what exists.