Short answers, pulled from the story.
Sejanus was a Roman soldier and confidant of Emperor Tiberius who lived from around 20 BC to the 18th of October, AD 31. As prefect of the Praetorian Guard from AD 14 until his execution, he transformed the guard from an imperial bodyguard into a powerful political institution and became, for a time, the most influential and feared citizen in Rome.
Sejanus consolidated the guard’s scattered encampments into a single garrison just outside Rome, the Castra Praetoria, in AD 20, and increased the number of cohorts from nine to twelve. He abolished the practice of shared command between two prefects and personally appointed the centurions and tribunes, giving him direct loyalty of around 12,000 soldiers.
Sejanus became sole commander of the Praetorian Guard in AD 15 when his father was reassigned to Egypt. By AD 23, Tiberius called him Socius Laborum, meaning my partner in my toils. When Tiberius withdrew to the island of Capri in AD 26, Sejanus was left in effective control of the imperial administration and controlled all information passing between the emperor and Rome.
Sejanus seduced Livilla, the wife of Tiberius’s son Drusus Julius Caesar, and with her as an accomplice had Drusus slowly poisoned. Drusus died of apparently natural causes on the 13th of September, AD 23. The truth emerged only after Sejanus’s own execution, when his ex-wife Apicata sent a letter to Tiberius naming Livilla, and Livilla’s slaves confirmed the poisoning under torture.
According to Josephus, Antonia, the mother of Livilla, sent a letter to Tiberius on Capri, carried by her freedman Pallas, warning him of the threat Sejanus posed. Tiberius then used a strategy of contradictory letters to confuse and isolate Sejanus before appointing Naevius Sutorius Macro to replace him. On the 18th of October, AD 31, Sejanus was lured to a Senate meeting, arrested, and strangled that same evening.
Sejanus became a recurring symbol of the overreaching favourite across European literature and politics. Ben Jonson wrote a play, Sejanus: His Fall, understood as a reference to Robert Devereux, 2nd Earl of Essex. Robert Graves included Sejanus in his 1934 novel I, Claudius, and the 1976 television adaptation cast Patrick Stewart in the role. The name was also applied to Cardinal Mazarin, the Duke of Buckingham, and Prime Minister Robert Walpole in political pamphlets spanning the 17th and 18th centuries.