Lucius Aelius Sejanus, from Etruria [see map of Italy], was the son of Seius Strabo; (an equestrian who was to become prefect of Egypt in A.D. 15, and then governor). Sejanus was a trusted advisor of the second Roman emperor, Tiberius, and himself the commander of the praetorian guard and, later, consul.
When Tiberius came to the throne, he appointed Sejanus praetorian prefect. Sejanus began to manipulate the praetorian guard into a personal guard for the emperor for efficiency and power.
In A.D. 20, Sejanus' daughter was betrothed to Claudius' son Drusus, but he died. In 22, Tiberius and his praetorians earned commendation for preventing the spread of the fire that burned Pompey's theater. When Tiberius' son Drusus died in 23, Sejanus -- who had quarreled with the emperor's son -- tried (unsuccessfully, because as an equestrian he was too low class) to marry his widow Livilla. Even without the marriage, he continued to amass power, and when Tiberius retired to Capri in 27, Sejanus was left as regent. Sejanus became consul with Emperor Tiberius in 31.
Sejanus tried to destroy the Julio-Claudian heirs and may have conspired with Livilla in the death of Drusus. Opposition to Sejanus resulted in charges of treason (maiestas) and Senate trials. The informers (delatores) received part of the estate of those accused of treason. Convicted victims of the informers were either executed or exiled. Sejanus' power came from two sources, the 9000 praetorians and the support of the emperor, but when Tiberius learned that Sejanus was plotting against him, he had Sejanus arrested and executed on October 18, 31 A.D. Following his death, the supporters and family of Sejanus were hunted down.
[See DIR - Tiberius]
Dio Cassius Book LVII
Suetonius - Life of Tiberius
Tacitus Books IV and V.
Tiberius, Second Edition, by Robin Seager; Malden, MA: Blackwell Publishing, 2005. Pp. xxvi, 310. ISBN 1-4051-1528-7.