Aelius Sejanus

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. Despite failing in an attempt to marry Drusus' widow 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 and had him 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.