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Chaos orcs are orcs that drank from Mannoroth's Chaos Well in Ashenvale Forest and became corrupted.
They were orcs of the Warsong clan that followed Grom Hellscream's decision to drink from the well to gain power in order to defeat Cenarius and his forces (Shadowleaves, Moonhunters, and Nightblades). By drinking from the well, they were further corrupted with more of Mannoroth's blood, turning red and growing larger.
Chaos orcs were capable of causing special damage known as Chaos damage. Chaos orcs were units seen in the Warcraft III: Reign of Chaos orc campaign.
Chaos orcs also fought under the banner of the Burning Blade during the First War. Some of the orcs who passed through the Dark Portal were driven mad, becoming chaos orcs and joining the Burning Blade.
Many chaos orcs were also seen serving the Horde after the Cataclysm as Dark Portal guardians. Sometime later, many joined Garrosh and his "True Horde".