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The sha are creatures native to Pandaria. They are the physical manifestations of negative emotions like fear, hatred, and anger. The arrival of the Alliance and the Horde and their imported conflict awakens the dormant Sha. The reason the pandaren are such a calm, peaceful people is to prevent their negative emotions manifesting as the sha.
When the Titans brought order to Azeroth, they slew a seven-headed Old God named Y'Shaarj. However, Y'shaarj's "last, terrible breath" cursed Pandaria with the shadowy aspects of the fallen Old God, the sha. For reasons unknown, the sha mostly lay dormant within Pandaria for many thousands of years, with only the lesser sha known to have ever manifested at this time.
Ten thousand years ago, Shaohao, the last Pandaren emperor, sought to save Pandaria from the impending Great Sundering. To protect his people, Shaohao undertook a grand quest to strip away his own negative emotions. He succeeded, and the doubt, anger, fear and still more that he plucked from himself was imprisoned beneath the earth. But it did not lie still. It seeped into the very land Shaohao hoped to protect. In the heart of Pandaria, Shaohao’s darkness – the Sha – waited, feeding off every harsh word, angry fist or pang of despair.
The Pandaren have trained themselves in compassion, patience, and love of life in order to limit their contribution to the power of the Sha, but with the arrival of violent outsiders on Pandaria, all their care may be for naught.It is not clear if the Sha have an agenda, but there is no doubt that they exist in disharmony with life. Wherever they lurk, they bring and delight in discord and death in a cycle that seems difficult for even the greatest heroes to end. Swords cannot slash doubt. Armor cannot deflect hatred.
Unwary mortals can also become possessed by negative Sha energy, becoming embodiments of pure anger or fear.Individuals turn black and white, and may gain other Sha-like features such as Sha arms or masks. It is possible to reverse Sha transformation in the beginning stages, as Rell Nightwind and Nazgrim were being possessed and were saved from such a fate by Taran Zhu. It is also possible for an individual to completely transform into a Sha, like Ga'trul and Doren do.
The Sha maintain a very big presence in the Dread Wastes, having possessed the Mantid Queen and most of her underlings, and the land itself became corrupted.
Recent developments in the Alliance-Horde War have led to attempts to harness the sha as a weapon against the opposing force, unaware of the potential danger it poses to its very users. After Garrosh arrived in Pandaria he began experimenting with the sha he found in the mountains of the north, believing that the sha can strengthen his soldiers. The results, however, have been quite the opposite. His troops are erupting into random violence, and it later emerges that the sha controls those who try to use it directly, rather than the other way round.
The sha's strength is mostly determined by their size; the larger the manifestation the more powerful they have become.Three appearances are used to represent them: lesser weak ones, medium strong ones, and large powerful ones. The latter are the 'main' sha aspects of negative emotions.
Most of the sha and the energies associated with them are monochromatic, the exception is Sha of Fear, which is a mixture of dark purple and red, and the Sha of Pride and its associated spawns, which are bit more blueish in tone instead of black/grey.
The sha minions created directly by the Heart of Y'Shaarj (both inside and outside his realm) are a bit more violet in tone in addition to their original monochromatic forms, signifying their infusion with the Old God's power.