quality:corruption, structural:reformatting
Tiberius in Capri, with Sejanus in Rome

Lucius Aelius Sejanus had served the imperial family for almost twenty years when he became Praetorian Prefect in AD 15. As Tiberius became more embittered with the position of Princeps, he began to depend more and more upon the limited secretariat left to him by Augustus, and specifically upon Sejanus and the Praetorians. In AD 17 or 18, Tiberius had trimmed the ranks of the Praetorian Guard responsible for the defence of the city, and had moved it from encampments outside of the city walls into the city itself, giving Sejanus access to somewhere between 6000 and 9000 troops.

The death of Drusus elevated Sejanus, at least in the eyes of Tiberius, who thereafter refers to him as his Socius Laborum (Partner of my labours). Tiberius had statues of Sejanus erected throughout the city, and Sejanus became more and more visible as Tiberius began to withdraw from Rome altogether. Finally, with Tiberius's withdrawal in AD 26, Sejanus was left in charge of the entire state mechanism and the city of Rome.

Sejanus's position was not quite that of successor; he had requested marriage in AD 25 to Tiberius's niece, Livilla, though under pressure quickly withdrew the request. While Sejanus's Praetorians controlled the imperial post, and therefore the information that Tiberius received from Rome and the information Rome received from Tiberius, the presence of Livia seems to have checked his overt power for a time. Her death in AD 29 changed all that.

Sejanus began a series of purge trials of Senators and wealthy equestrians in the city of Rome, removing those capable of opposing his power as well as extending the imperial (and his own) treasury. Germanicus's widow Agrippina the Elder and two of her sons, Nero Julius Caesar and Drusus Caesar were arrested and exiled in AD 30 and later all died in suspicious circumstances. In Sejanus's purge of Agrippina the Elder and her family, Caligula, Agrippina the Younger, Julia Drusilla, and Julia Livilla were the only survivors.

In 31, Sejanus held the consulship with Tiberius in absentia, and began his play for power in earnest. Precisely what happened is difficult to determine, but Sejanus seems to have covertly attempted to court those families who were tied to the Julians and attempted to ingratiate himself with the Julian family line to place himself, as an adopted Julian, in the position of Princeps, or as a possible regent. Livilla was later implicated in this plot and was revealed to have been Sejanus's lover for several years.

The plot seems to have involved the two of them overthrowing Tiberius, with the support of the Julians, and either assuming the Principate themselves, or serving as regent to the young Tiberius Gemellus or possibly even Caligula. Those who stood in his way were tried for treason and swiftly dealt with.

In AD 31 Sejanus was summoned to a meeting of the Senate, where a letter from Tiberius was read condemning Sejanus and ordering his immediate execution. Sejanus was tried, and he and several of his colleagues were executed within the week. As commander of the Praetorian Guard, he was replaced by Naevius Sutorius Macro.

Tacitus claims that more treason trials followed and that whereas Tiberius had been hesitant to act at the outset of his reign, now, towards the end of his life, he seemed to do so without compunction. The hardest hit were those families with political ties to the Julians. Even the imperial magistracy was hit, as any and all who had associated with Sejanus or could in some way be tied to his schemes were summarily tried and executed, their properties seized by the state. As Tacitus vividly describes,

"Executions were now a stimulus to his fury, and he ordered the death of all who were lying in prison under accusation of complicity with Sejanus. There lay, singly or in heaps, the unnumbered dead, of every age and sex, the illustrious with the obscure. Kinsfolk and friends were not allowed to be near them, to weep over them, or even to gaze on them too long. Spies were set round them, who noted the sorrow of each mourner and followed the rotting corpses, till they were dragged to the Tiber, where, floating or driven on the bank, no one dared to burn or to touch them."

However, Tacitus's portrayal of a tyrannical, vengeful emperor has been challenged by some historians: Edward Togo Salmon notes in A History of the Roman World:

"In the whole twenty two years of Tiberius's reign, not more than fifty-two persons were accused of treason, of whom almost half escaped conviction, while the four innocent people to be condemned fell victims to the excessive zeal of the Senate, not to the emperor's tyranny."

While Tiberius was in Capri, rumours abounded as to what exactly he was doing there. Suetonius records the rumours of lurid tales of sexual perversity, including graphic depictions of child molestation, cruelty, and most of all his paranoia. While heavily sensationalised, Suetonius's stories at least paint a picture of how Tiberius was perceived by the Roman senatorial class, and what his impact on the Principate was during his 23 years of rule.