Old God

The Old Gods are eldritch parasitic horrors which were created by the Void Lords. The Void Lords desired to warp the universe into a realm of chaos and suffering. Opposing them were the Titans, planet-sized cosmic beings who spread life across the cosmos. They created the Old Gods to corrupt nascent Titans before they awakened.

From the Void, the Old Gods were hurled into the universe, in the hope that some of them would smash into worlds containing sleeping Titans. Their sole purpose being to locate and corrupt the sleeping Titan world-souls. The Old Gods being the physical manifestations of the Void, appear as colossal horrors, growing on those worlds like cancerous tumors.

Several of them ended up on Azeroth where they established the Black Empire, spawned two servitor race, and ruled the planet. They were later defeated by the Titans' armies and imprisoned. Despite their sequestration, they were able to influence the world, corrupting races and individuals. Originally they waged war against each other, but ever since their imprisonment they have been working together, influencing the history of Azeroth.

The greatest of their servants during their imprisonment was the great dragon, Deathwing who lived for over 10,000 years, led a flight of corrupted dragons, and was responsible for the recent global cataclysm which reshaped the world. Another great monster in their service is Ozumat, the patriarch of the krakens. Queen Azshara is another one, the queen of the night elfs who betrayed her people and attempted to summon to Azeroth the dark titan Sargeras, Lord of the Burning Legion. In the aftermath of her failure and the flooding of her city, she made a pact with the Old Gods, as she and her followers were transformed into the naga.

Old Gods

 * Y'Shaarj: An Old God of Seven Heads. The greatest and most evil of the Old Gods. It was known as the "Beast of the Seven Heads", a being which inhaled courage and breathed fear. Unlike the other Old Gods, it was killed by the Titans. Upon it's death, it's essence gave rise to the Sha.
 * C'Thun: An Old God of Chaos and Madness. An incomprehensibly evil entity whose malevolence has plagued Azeroth. It spawned the insectoid race of the Aqir and recently broke free from its imprisonment, though it was defeated by the combined forces of the Horde and the Alliance.
 * Yogg-Saron: An Old God of Death which was imprisoned in the continent of Northrend, within a Titan complex. Though imprisoned it has been responsible for many horrors that have plagued Azeroth including the Curse of Flesh and the Emerald Nightmare.
 * N'Zoth: The weakest of the known Old Gods and an Old God of Madness. Which had been imprisoned in the deepest depths of the ocean. Of all the Old Gods, it was the one that scholars and priests knew least about. It's most infamous achievement was corrupting a group of highborn night elves which it transformed into the Naga.
 * G'huun: An Old God of Blood.

Minions
The greatest of the Old Gods minions are the N'raqi, an intelligent and cunning race of nearly unstoppable juggernauts who are fanatically loyal to the Old Gods. A monstrous race which originated from the organic matter that seeped from the Old Gods. Most of them were imprisoned alongside the Old Gods, and those that weren't were hidden in the isolated parts of the world.

From Y'Shaarj came the Sha, malevolent spirits which were created upon its death by its essence. They were physical manifestations of various negative emotions. Due to the Pandaren Song, they mostly slumbered for 1000s of years in the continent of Pandera, with only the lesser ones being active. Recently due to the wars brought to Pandera by the Alliance and the Horde, they have awakened.

The Aqir are an insectoid race which was spawned by the organic matter from the Old Gods. An incredibly strong and resilient race which was employed as a labor force by the N'raqi. Upon the fall of the Black Empire, they went underground and did not reemerge till millennia later. They waged a genocidal war to exterminate all non-insectoid lifeforms and were defeated. After which they broke up into three insectoid races; the Mantids (preying-mantis like insectoids), the Qiraji (multi-form insectoids), and the Nerubians (spider-like centaurids) whom continue to worship and serve the Old Gods.