Locusta came from the Roman province of Gaul and became the most notorious maker of poison in the first century. By AD 54 she was already sitting in a Roman prison on poisoning charges, which made her a convenient asset for the Empress Agrippina the Younger, who wanted her husband Claudius dead so that her son Nero could take the throne ahead of Britannicus. Drawing on Tacitus, Suetonius, Cassius Dio, and the satirist Juvenal, this episode traces how a captive expert in deadly nightshade came to shape the succession of the Julio-Claudian dynasty.

Her story is one of coercion as much as cunning. When her first dose failed to kill Britannicus in AD 55, Nero flogged her with his own hands and threatened to execute her on the spot. A stronger formula worked before dinner was over, and the terrified prisoner was pardoned, handed rich country estates, and sent pupils to train. That protection lasted only as long as Nero did. The episode asks whether history’s wicked poisoner was the villain, or simply the most effective weapon in a room full of ruthless rulers.

• Locusta’s specialty was Atropa belladonna, whose alkaloids cause a racing heart, dilated pupils, delirium, and finally respiratory failure.
• The poison for Claudius was hidden in a dish of mushrooms, and when it worked too slowly the doctor Xenophon reportedly finished the job with a poisoned feather.
• Britannicus was killed with cold water added to cool a drink his taster had already approved, and Nero told the guests it was an epileptic fit.
• In AD 68 a collapsing Nero asked Locusta for one last poison for himself, carried it in a golden box, and never used it.
• The new emperor Galba had her stripped of her estates, led in chains through Rome, and publicly executed alongside Nero’s favored freedmen.