Tiberius Julius Caesar Nero, known as Tiberius Gemellus (10 October AD 19 – 37/38), was the son of Drusus and Livilla, the grandson of the Emperor Tiberius, and the cousin of the Emperor Caligula. Gemellus is a nickname meaning "the twin". His twin brother, Germanicus Gemellus, died as a young child in AD 23. His father and older cousins died, and are suspected by contemporary sources as having been systematically eliminated by the powerful praetorian prefect Sejanus. Their removal allowed Gemellus and Caligula to be named joint-heirs by Tiberius in 35, a decision that ultimately resulted in Caligula assuming power and having Gemellus killed (or forced to kill himself) in late 37 or early 38.
Background
Gemellus was born the son of Drusus Julius Caesar and Livilla on 10 October AD 19, the same day his uncle Germanicus died in Syria. His paternal grandparents were emperor Tiberius and his wife, Vipsania Agrippina, with his maternal grandparents being general Nero Claudius Drusus and Antonia Minor. He was the twin brother of Germanicus Gemellus and the younger brother of his sister Julia. The birth of Gemellus and his twin were celebrated by Tiberius, who claimed that never before in the history of Rome had twins been born to a man as high in rank as Drusus, and the event was commemorated on the reverse of coins. His twin died while still a child in 23.
As a member of the Julio-Claudian dynasty, Gemellus was a close relative of all five Julio-Claudian emperors. On his father's side, he was the grandson of emperor Tiberius, and his adoptive great-grandfather was the founder of the Roman Empire, Augustus. On his mother's side, he was also Augustus' great-grandnephew, the nephew of Claudius, the cousin of Caligula, and the first cousin once removed of Nero.
Life
Downfall of his relatives
In the spring of 22, his father Drusus received tribunicia potestas (tribunician power) from the Senate, a clear sign that Drusus was Tiberius' heir. However, the following year marked a turning point for his father when his dispute with the powerful praetorian prefect Lucius Aelius Sejanus reached a critical point. In the account of Tacitus, Sejanus began plotting against Drusus to secure his position. On 14 September 23, his father died from what passed as natural causes. According to Cassius Dio and Tacitus, his father was poisoned by his wife Livilla at the behest of Sejanus. The exact cause of the feud is unknown.
His father's death not only devastated Tiberius but also challenged the future of the principate. Tiberius still trusted Sejanus and had no suspicion. Since Gemellus was too young, Tiberius adopted his great-nephews by Germanicus, Nero and Drusus, and recommended them to the Senate. Nero was given the office of quaestor five years in advance and was married to Gemellus' sister Livia to combine the families of both possible successors. However, neither would live to succeed Tiberius.
By 26, the emperor had withdrawn from politics altogether and moved to Capri, leaving the management of the empire to Sejanus who then began eliminating other members of the imperial family. In 28/29, Nero was charged by the Senate with homosexuality for which he was exiled to the island of Ponza. Germanicus' son Drusus was imprisoned within the dungeon under the Imperial palace on the Palatine Hill, where he starved to death not long after. Nero died in exile in 33. Suetonius says Tiberius promoted the rise of Sejanus to secure the succession of Gemellus, his natural grandson, at the expense of Germanicus' sons Nero and Drusus.
Sejanus remained powerful until his sudden downfall and summary execution in October 31, the exact reasons for which remain unclear. On 26 October 31, just eight days after his death, his wife Apicata committed suicide. According to Cassius Dio, she left a message for the emperor: his son had been murdered by his wife Livilla and her lover, Sejanus. The story should be read with caution. Barbara Levick says that Sejanus must have murdered Drusus in self-defense because only Tiberius stood between the Praetorian Prefect and the end of his career at the hands of Drusus. Furthermore, she says it is even less likely that Livilla would have been complicit in the destruction of her family, the key to her children's future. Levick dismisses the accusation of Apicata as the revenge of a woman whose husband left her for another.