The Metacon War was a war fought between the Protheans and an unknown, hostile artificial lifeform. The exact dates of this war have been lost, but it is known to have occurred some time between 68,000 BCE and 48,000 BCE. It is believed to have been early in the development of the empire the Protheans built, and indeed spurred much of its expansion as the Protheans sought to unite all of the galaxy's organic sapient species against this synthetic foe.
Ultimately, the Protheans proved victorious.
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The Metacon War is derived from Mass Effect.