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A Blood Curse (or the blood-curse and also referred to as the blood pact) is the curse laid upon orcs when they consume the blood of a demon. Gul'dan had persuaded the highest-ranking orcs of Draenor to drink demonic blood. It was this stain that had hounded them for so long. Even those who had not partaken found themselves developing an unquenchable thirst for slaughter, their skin turning green with the taint.
Orcs that drink the blood of a pit lord were shown gaining supernatural strength, speed, resilience and stamina. The effects as well as side effects are immediate, and the symptoms that manifest from blood consumption are green skin, glowing red eyes and a considerable increase in size. Further exposure and physical change cause the skin to turn red, a greater increase in body mass, green/purple/yellow eyes, extra tusks and bone growth that breaks the skin.
The demon blood most of them had imbibed back on Draenor had increased their natural bloodlust a hundredfold. The orcs, their systems permeated with demonic blood, burned with a lust for killing that would not be sated even if they slew every last draenei on Draenor, would never be sated until death stiffened their corpses.
Lethargy of the orcs
Eventually cut off from their sustenance, the orcs slipped into a state of lethargy. Antonidas learned that the orcs had been under the crippling influence of demonic power (or warlock magics) for generations. He speculated that the orcs had been corrupted by demonic powers even before their first invasion of Azeroth. Clearly, demons had spiked the orcs' blood, which in turned granted the brutes unnaturally heightened strength, endurance and aggression. Antonidas theorized that the orcs' communal lethargy was not actually a diseases, but a long-term racial withdrawal from the volatile warlocks magics that had made them fearsome, blood-lusted warrior. Though the symptoms were clear, Antonidas was unable to find a cure for the orcs' present condition. Many of his fellow mages, as well as a few notable Alliance leaders, argued that finding a cure for the orcs would be an imprudent venture. Left to ponder the orcs' mysterious condition, Antonidas' conclusion was that the orcs' only cure would have to be a spiritual one.
Cenarius, as a being so attuned to nature that he can sense the slightest corruption, assumed that the orcs in Ashenvale were scouts of the Legion. This, ironically, sent the Warsong clan back into the service of Mannoroth and lead to the reestablishment of their connection to the potent fel magics that first bound them to the Legion. Thrall was able to purge the curse from Grom Hellscream using a Soul Gem and an elven Ritual Circle. Together they faced Mannoroth. It had been Grom who had first drunk the blood of the demon Mannoroth - blood that had tainted him and all others orcs who had drunk it. But in the end, Grom had had his revenge. Though he had been the first to drink, and thus the first to fall to demonic bloodlust and madness, he had been the one to end the curse. He had slain Mannoroth. And with that gesture, the orcs had begun to reclaim their own great hearts, wills, and spirits. However, their skin remains green, due to the lingering effects of fel exposure.
The Blood Curse enslaving the orcs to the Legion may function based on the law of sympathy. Being a part of the demon's body, blood would have a very strong sympathy and link the imbiber to the source of the blood. Through this link, a powerful demon could influence and control the imbibers. When that demon is killed, the sympathy is broken and they become free, as we saw with the orcs after Mannoroth's death.