The Aelius Sejanus reference article from the English Wikipedia on 24-Apr-2004
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Aelius Sejanus

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Lucius Aelius Sejanus (d. 31 AD) was a praetorian prefect of the Roman Empire, and for a time the most influential citizen of Rome.

Sejanus was appointed praetorian prefect on the accession of Tiberius, as the colleague of his father L. Seius Strabo. On his father's appointment to the governorship of Egypt, he became sole commander of the Praetorian Guard, and began to increase his power.

After the death of Tiberius' son Drusus in 23, he consolidated his power over the Senate, and concentrated the Praetorian guard in a single camp outside the Viminal Gate of Rome. Despite failing in an attempt to marry Drusus' widow Livilla in 25, which would make him part of the ruling house, he increased his power further upon Tiberius' retirement to Capri in 27.

In 31, when he gained the consulship, Sejanus felt his position was unassailable, and plotted to seize the purple for himself. Tiberius discovered the plot, however, and Macro, prefect of the Vigiles, arrested him. When Sejanus was executed, Macro succeeded him as commander of the Praetorian Guard.

Reference

Sejanus' fall from power is recounted in detail by Roman historians, particularly Gaius Cornelius Tacitus.