Locusta (or Lucusta) was a notorious maker of poisons in Ancient Rome.
She was said to come from Gaul. In AD 54, already notorious and imprisoned on poisoning charges, Locusta was ordered by the empress Agrippina the Younger to supply a poison for the murder of her husband, Claudius. This was sprinkled on a mushroom and given to the emperor by his food-taster Halotus; when this poison appeared to be ineffectual, the doctor Xenophon murdered Claudius with a poisoned feather ostensibly put down his throat to induce vomiting.
In AD 55, while still imprisoned, Locusta was called upon by Agrippina's son, the emperor Nero, to concoct a poison to murder Claudius' son Britannicus. When this poison was slow to work, Nero flogged Locusta with his own hand and threatened her with immediate execution, whereupon she supplied a quicker-acting poison that succeeded. Nero rewarded Locusta with a full pardon and large country estates where he sent pupils to learn her craft. Before Nero fled Rome in AD 68, he acquired poison from Locusta for his own use and kept it in a golden box, but died by other means
After Nero's suicide, Locusta was condemned to die by the emperor Galba during his brief reign, which ended 15 January AD 69. Along with Nero's favorites Helius, Patrobius, Narcissus and “others of the scum that had come to the surface in Nero's day,” she was led in chains through the city and executed.
Juvenal refers to her in one of his Satires, describing a poisoner even more skilled than Locusta.
In The Count of Monte Cristo by Alexandre Dumas, the poisoner Madame de Villefort is frequently compared to Locusta. Chapter 101 is entitled “Locusta”.