Locusta

Locusta was a professional poisoner in Rome who also poisoned others in her spare time. She thought to be the first documented serial killer. In A.D. 54, she was hired by Agrippina the Younger to kill the Emperor Claudius, which she accomplished with a poisoned dish of mushrooms. In 55, she was convicted of poisoning another victim. When Nero learned of this he sent a tribune of the Praetorian Guard to rescue her from execution. In return for this she was ordered to poison Britannicus. She succeeded on her second try, Nero rewarding her with immunity from execution while he lived. Seven months after Nero's suicide, Locusta was condemned to die by Galba on January 69.

The Doctor, Barbara Wright, Ian Chesterton, and Vicki encountered her in the year 64.