A Blood Curse, also known as the Blood Pact, is when an orc or group of orcs consume the demonic blood of a powerful pit lord to gain 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.
Orcs under the Blood Curse are in a constant state of aggression and violence, and maintaining normal composure requires considerable self-control.
A Blood Curse can even affect those who do not consume demonic blood; even though only twelve clan chieftains personally drank Mannoroth's blood, nearly every orc in the Horde was soon under its influence.
The effects of the curse are not limitless in duration, however. Eventually, if not given continuous reinforcement, it fades and leaves the user in a state of withdrawal.
Pit Lords used for a Blood Curse
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.