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Lucius Aelius Sejanus was the Emperor of the Magna Roman Empire. He was responsible for overthrowing Tiberius and then cleansing the Empire of Christians.
In 2364, the crews from the USS Centurion and USS Enterprise-D compared the point in history where Human and Magna Roman history diverged. They determined it was when Lucius Sejanus overthrew Tiberius and his Human counterpart failed and was then executed. (TNG novel: The Captains' Honor)