Lucius Aelius Sejanus was the son of Tiberius' first Praetorian Prefect Strabo. An equestrian by birth, he had connection to the Imperial family almost his entire life, through the service of his father. In 16 AD Strabo was appointed the governorship of Egypt (the highest political position for an Equestrian of the time), and Sejanus moved fluidly into the command of the Praetorian Guard. Likely around 20 years old, (the exact date of his birth is unknown) he was quickly becoming one of Tiberius' closest confidants and trusted advisors. This relationship would immediately put him at odds with other members of the Imperial family, including the emperor's son, Drusus.
Within a couple of years of his sole appointment as Praetorian Prefect, Sejanus solidified his position, and his command, by concentration the previously scattered Guard all within easy reach Rome. The 9,000 men of the Praetorian cohorts were no longer a force charged with keeping the peace around the Italian towns, but were truly the emperor's personal guard. By virtue of the size of this command, the Prefect undoubtedly became a very pronounced figure in the Roman system of government and daily affairs. Tiberius himself dubbed Sejanus as his "partner of my labors", and while this may have been initially true, Sejanus wouldn't take long to advance his own agenda.
Probably very early on, Sejanus developed a rivalry with Tiberius' son Drusus. When Germanicus, heir to Tiberius, died in 19 AD, this rivalry would take a noticeable turn for the worse. Sejanus was likely beginning to view himself as a potential heir, and Drusus was the one man who stood in his way. Over the next few years, Sejanus impressed Tiberius through his many administrative abilities and the young Prefect continued to be endowed with more power. By 23 AD, Drusus died after a short but violent illness and the way was opened for Sejanus to take an even more prominent role. Later accusations of poisoning would develop due to treason trials resulting in and from Sejanus fall. Sejanus and Drusus wife Livilla were accused of adultery and conspiracy in Drusus death. However, the truth of the matter is completely unknown, and the entire incident is deeply debated by modern scholars.
After the death of Drusus, Agrippina the Elder, wife of Germanicus and granddaughter of Augustus, had already been, but became an even more important political player. Through various political schemes, for Agrippina this included advancing her sons Nero, Drusus and eventually Gaius (Caligula) into positions as Tiberius' heir. At the same time, Sejanus' took more and more of a prominent role in Roman politics. A dangerous enmity developed that would threaten the very life of the Julio-Claudian line. However, at 64 years old, and perhaps always yearning for a life of solitude away from Rome (as evidenced by his withdrawal from public life at the turn of the millennium), Tiberius simply wanted someone to take over for him and act as regent. Agrippina's sons were still too young (ranging in the teens) and Sejanus presented the best chance for Tiberius' 'escape'. According to Tacitus, Tiberius even longed for a return to Republican rule, but this was simply unattainable. Instead, Tiberius retired from public life again and withdrew to the completely isolated island of Capri in 26 AD. Sejanus was left in charge as regent, and he would soon use that new power to advance his own personal agenda and nearly completely destroy the Julio-Claudian line.
After Tiberius' withdrawal, in which he would never return to Rome, Sejanus systematically took control of the government. The family of Agrippina was relentlessly attacked, mainly through her friends and supporters in treason courts. However, she and her sons fell victim as well. Agrippina and her oldest son Nero were arrested perhaps as early as AD 27. In 29 AD they were exiled to the Pontian islands off the coast of Naples and the masses who had always favored Germanicus, protested in the streets. Agrippina's second son, Drusus, was arrested in 30 AD but not exiled. Nero was forced into suicide in 31 AD and within two more years both Agrippina and Drusus had died of starvation. Meanwhile, any in the Senate who mounted opposition to Sejanus in any form, found themselves in terrible danger of the treason courts. Sejanus also controlled access to Tiberius, whose position on Capri made him virtually inaccessible. The Senate had little choice but to cow to the man who controlled 9,000 Praetorians within the very walls of Rome. However, Gaius (Caligula) the eventual heir remained safely in Tiberius' direct care (and protection) on Capri.
Sejanus controlled all matters of Roman administration during this period. From military matters to political appointments he truly wielded the ultimate power. Sejanus' ultimate goal, whether he sought preservation through being the trusted member of the Imperial staff, or truly intended to claim ultimate power for himself, is unfortunately unknown. This issue too has been widely debated both among ancient contemporaries and in modern study. Sejanus did attempt to marry Livilla, the wife of Tiberius' son Drusus, but Tiberius blocked this as a measure of social conformity (Sejanus was still an equestrian and Livilla was a member of a noble family of the highest order). However, by 30 AD, Tiberius seemingly withdrew any public opposition to Sejanus' climb. Sejanus was betrothed to Livilla's daughter, and by 31 AD, he was named joint consul with Tiberius (an honorary compliment indicating that he was the official heir). In that same year, he was granted an additional share of the emperor's proconsular power, yet another step to the ultimate power. However, many argue that Tiberius was just keeping Sejanus in check and as unsuspicious as possible, as he was already suspected of treason.
Likely in 31 AD, Tiberius received a letter, which somehow managed to get through Sejanus' web of spies, from his widowed sister-in-law Antonia. She was completely within Tiberius' trust, perhaps because she had little involvement in political affairs. In her letter, she accused Sejanus of a plot to seize power, and Tiberius, whether he already suspected Sejanus or not, now began to act in a matter which confirms this belief. The emperor began feeling out the Senate and other Equestrians in postions of authority. Mixed messages regarding Sejanus' position were given in private, while in public, he continued to lavishly praise the man he called 'my Sejanus'. Coins were minted in his honor and statues erected, but Caligula meanwhile, began to be propped up in public as well. When Tiberius granted Sejanus proconsular power, Caligula was made Consul giving the appearance that perhaps he might yet play a part in the game of succession. Meanwhile too, other more mild attempts to keep Sejanus off balance were undertaken. Tiberius also learned, much to his relief, that the Praetorians were still loyal to him and only supported Sejanus because he was Tiberius' regent.
Now satisfied in his own security and that of his remaining family, Tiberius began the process of removing the powerful Sejanus (the man he had granted that power to in the first place). Q. Sutorius Macro was made the new commander of the Praetorian Guard and Tiberius sent him with a letter to be read before the Senate. In this letter, it was expected that Sejanus would be given tribunician power, much like the emperor himself, thereby making him co-emperor and the obvious choice for succession. However, Tiberius had another idea altogether in mind. First Macro made sure of the support of the Praetorians who were present, and in the barracks. While he was doing this, the incredibly long and rambling, yet supportive, letter of Tiberius was read before the Senate, and in the presence of Sejanus. Just as they thought it was going to offer Sejanus the power he craved, the letter changed tone abruptly scathingly denouncing his right hand man and ordering his arrest.
On October 18, 31 AD, Sejanus was arrested and the people of Rome celebrated. However, the men of the Senatorial elite were understandably panicked. Each had given support to Sejanus in some way, how could they not? Tiberius himself granted the man near imperial authority. That very same day, the man who seemed to be the front runner to become the third Roman emperor was executed and the order of things couldn't take such a dramatic swing without terrible repercussions. A veritable witch hunt of Sejanus' supporters was to commence, in which his family was largely executed. Livilla, the former wife of Tiberius' son Drusus, was executed for her role in that affair. His friends were denounced, tried and executed, or forced into suicide or exile. Tiberius attempted to convey that Sejanus was implicated for many reasons including plots against various family members. However, Agrippina and Drusus who still remained alive in imperial custody were allowed to rot away, experiencing unspeakable tortures for another 2 years after Sejanus' fall. Though the ancient sources, notably Tacitus, paint a gruesome picture of the year 33 AD, in which a final purge was to occur, some argue that Tiberius was actually quite just in his approach. However, to the aristocracy which would live on to tell the tale, Tiberius was a fool who allowed all of this to happen in the first place, and history remembers both he and Sejanus as evil men.