Drusus the Younger, born in 13 BC, was the son of the emperor Tiberius and Tiberius' first wife Vipsania Agrippina. Drusus was married, to Livia Julia, a daughter of Tiberius' dead brother, Drusus the Elder, and thus the younger Drusus' own cousin. Drusus the Younger and Livia Julia had children, including twin brothers named Tiberius Gemellus and Germanicus (born AD 19).
As of AD 23, all indications were that Drusus the Younger was Tiberius' choice to succeed him as emperor when Tiberius died or retired. (Born in 42 BC, Tiberius was in his mid-sixties.) However, Tiberius expected that Drusus would in effect serve as a caretaker - but not for his own sons. When the time came, Drusus would be succeeded as emperor by one of the sons of his dead cousin Germanicus (15 BC - AD 19, a son of Drusus the Elder) and Germanicus' surviving wife Agrippina the Elder, a granddaughter of the emperor Augustus. The eldest of these boys, Nero (born AD 6; not the future emperor) and Drusus (born AD 7) were in their teens. (There was also a third brother, Gaius, born AD, who later became known as the emperor Caligula, but that's another story.) In this way, the bloodline of Augustus would ultimately regain the principate, because Nero and Gaius were Augustus' great-grandchildren through Agrippina the Elder.
Obviously, Tiberius could not insure that Drusus would follow his wishes after Tiberius' died or stepped down, and cast aside his own sons in favor of one of his nephews. However, Drusus had provided indications that he was prepared to honor his father's request. At all events, it appeared more and more likely that within a year or two Tiberius would put aside the cares of office in favor of his son.
It was not to be. On September 14, AD 23, Drusus died at the age of 36. At the time there was no hint of foul play. Drusus had been something of a high-liver, and Tiberius and everyone else were convinced that Drusus had died of natural causes.
Eight years later, however, a foul conspiracy was uncovered: the pretorian prefect Sejanus had poisoned Drusus as part of a plot to assume power. Between the early 20s and AD 31, Sejanus had risen far indeed, becoming Tiberius' number one aide and virtual alter ego before Tiberius renounced him that year. He was executed on October 18, AD 31. Six days later his eldest son by his former wife, Apicata, whom Sejanus had divorced in early AD 23, was executed as well. Two days after that Apicata committed suicide.
Before her death, however, Apicata dropped a bombshell. According to Cassius Dio, she composed a letter to Tiberius revealing that Sejanus had seduced Drusus' wife Livia Julia and that the two of them had then conspired to murder Drusus:
[Sejanus'] wife Apicata was not condemned, to be sure, but on learning that her children were dead, and after seeing their bodies on the Stairway, she withdrew and composed a statement about the death of Drusus, directed against Livilla, his wife, who had been the cause of a quarrel between herself and her husband, resulting in their separation; then, after sending this document to Tiberius, she committed suicide. It was in this way that Tiberius came to read her statement; and when he had obtained proof of the information given, he put to death Livilla and all the others therein mentioned. I have, indeed, heard that he spared Livilla out of regard for her mother Antonia, and that Antonia herself of her own accord killed her daughter by starving her. These events, however, were later.
Subsequent investigation, including interrogation of the doctor and eunuch used by Sejanus to effect the murder (see below), confirmed the accusations. Sejanus had turned Livia Julia presumably by pointing out that, if Drusus became emperor, he would insure that one Germanicus' sons would succeed him to the throne rather than her own son, Tiberius Gemellus. With Drusus dead, however, Sejanus and Livia Julia could marry, and Sejanus would see to it that Tiberius Gemellus would eventually became emperor. Sejanus, of course, would be the power behind the throne. Time was of the essence, because Germanicus' sons were rapidly approaching manhood.
Having joined Livia Julia to his scheme, Sejanus then enlisted agents to poison Drusus in a way that would mimic a natural death. As Robin Seager summarizes in his biography Tiberius:
Livia Julia's doctor, Eudemus, was made privy to the plot, and Seianus divorced his wife Apicata . . .. The need for swift action had been underlined at the beginning of 23, when young Drusus [Germanicus' son] assumed to the toga uirilis. The same honors were bestowed on him as had been voted to Nero [Germanicus' eldest son; again, not the future emperor] three years before. . . . For his weapon, Seianus chose poison, the effects of which might be passed off as the symptoms of a disease. The draught was administered by a eunuch, Lygdus, probably Drusus' taster. Drusus was ill for several days, during which time Tiberius characteristically continued to attend all meetings of the senate, but on 14 September [AD 23] he died. The first stage of Seianus' grand design had been successfully completed.
The ancient sources took it us incontestable that Sejanus had, in fact, poisoned Drusus. Here, for example, is Tacitus (Annals, Book IV):
There were however obstacles to [Sejanus'] ambition in the imperial house with its many princes, a son in youthful manhood [Drusus the Younger] and grown-up grandsons [the teenagers Nero and Drusus]. As it would be unsafe to sweep off such a number at once by violence, while craft would necessitate successive intervals in crime, he chose, on the whole, the stealthier way and to begin with Drusus [the Younger], against whom he had the stimulus of a recent resentment. Drusus, who could not brook a rival and was somewhat irascible, had, in a casual dispute, raised his fist at Sejanus, and, when he defended himself, had struck him in the face.
On considering every plan Sejanus thought his easiest revenge was to turn his attention to Livia, Drusus's wife. She was a sister of Germanicus, and though she was not handsome as a girl, she became a woman of surpassing beauty. Pretending an ardent passion for her, he seduced her, and having won his first infamous triumph, and assured that a woman after having parted with her virtue will hesitate at nothing, he lured her on to thoughts of marriage, of a share in sovereignty, and of her husband's destruction. And she, the niece of Augustus, the daughter-in-law of Tiberius, the mother of children by Drusus, for a provincial paramour, foully disgraced herself, her ancestors, and her descendants, giving up honour and a sure position for prospects as base as they were uncertain. They took into their confidence Eudemus, Livia's friend and physician, whose profession was a pretext for frequent secret interviews. Sejanus, to avert his mistress's jealousy, divorced his wife Apicata, by whom he had had three children.
Sejanus accordingly thought that he must be prompt, and chose a poison the gradual working of which might be mistaken for a natural disorder. It was given to Drusus by Lygdus, a eunuch, as was ascertained eight years later. As for Tiberius, he went to the Senate house during the whole time of the prince's illness, either because he was not afraid, or to show his strength of mind, and even in the interval between his death and funeral. Seeing the consuls, in token of their grief, sitting on the ordinary benches, he reminded them of their high office and of their proper place; and when the Senate burst into tears, suppressing a groan, he revived their spirits with a fluent speech. "He knew indeed that he might be reproached for thus encountering the gaze of the Senate after so recent an affliction. Most mourners could hardly bear even the soothing words of kinsfolk or to look on the light of day. And such were not to be condemned as weak. But he had sought a more manly consolation in the bosom of the commonwealth."
Likewise Suetonius in his Life of Tiberius takes Drusus' murder as a given:
[Tiberius] increased his cruelty and carried it to greater lengths, exasperated by what he learned about the death of his son Drusus. At first supposing that he had died of disease, due to his bad habits, on finally learning that he had been poisoned by the treachery of his wife Livilla and Sejanus, there was no one whom Tiberius spared from torment and death.
So did it really happen this way? Was Drusus, in fact, poisoned? While admitting that "[c]ertainty is impossible," Robin Seager maintains "[t]hat Drusus died a natural death . . . seems to me certain." In support of his position, he raises a number of arguments.
First, "[i]t is striking that" for eight years "there had never existed the slightest suspicion that Drusus had not died a natural death," particularly "in a poison-conscious age." There was widespread suspicion that Drusus' own father Germanicus had been poisoned only four years earlier. And yet neither Drusus on his deathbed nor anyone else ever thought of poisoning until Apicata sent her letter. Tiberius himself "remained convinced for eight years that his son had died of ill-health and excessive self-indulgence," an entirely reasonable conclusion given his lifestyle.
Second, Seager finds it "almost incredible that Seianus and Livia Julia should have left Eudemus and Lygdus alive to tell their tale." They had eight years in which to liquidate the witnesses. Their failure to do so argues powerfully "against the presumption of murder."
Finally, Seager points out that it is hard to understand how Apicata would even have been aware of the plot. Certainly neither Sejanus nor Livia Julia would have told her. Assuming that Apicata had taken up with Livia Julia before divorcing her, the more likely scenario is that the resentful Apicata came to sincerely believe that Drusus had been murdered based upon that fact and her familiarity with Sejanus' ambition.