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Battle of Cleveland
Lepidus was the first member of the Covenant to hear of the Key of Osanalan after torturing Colonel James Ackerson. He then informed the High Prophet of Truth of the Key, which sparked the Battle of Cleveland. He then reported to the High Prophet of Truth aboard the Forerunner Dreadnought, where the Master Chief attempted to assassinate Truth.
Truth successfully escaped, as did Lepidus, but every Covenant unit who stayed in the room was killed by John-117. Lepidus later returned aboard the Triumphant Declaration in the orbit of Mars, furious at Colonel James Ackerson for tricking the Covenant into believing in the Key of Osanalan. He then beheaded Ackerson with his combat knife.
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