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. When Tiberius' son Drusus died in 23, Sejanus 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 co-consul with 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). The informers (delatores) received part of the estate of those accused of treason. 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.
