Old Gods in the Warcraft Universe are horrors created by the void lords. They are immense mountains of flesh, physical manifestations of dark shadow magic called the Void. Their sole purpose is to corrupt a titan world-soul into a being of unimaginable evil, a "warped titan" which would consume all matter and energy in the universe, bringing all of existence under the void lords' will.
Four Old Gods — Y'Shaarj, C'Thun, Yogg-Saron and N'Zoth — ruled the planet of Azeroth during its primordial age.
They have psychic power and have been whispering to the subconsciousness of beings at the surface, subverting their thoughts and feelings, and sometimes driving them to great malice, complete insanity, or both. The old whisperings urge one to do dark, terrible things. They are subtle whispers that eventually become indistinguishable from one's own thoughts.