Locusta

Locusta was a professional poisoner in Rome.

Biography
Vicki met Locusta whilst she and the First Doctor, who was impersonating Maximus Pettulian, were in the Nero's palace. Locusta created a poison, intended by Poppaea, to be used on Barbara Wright. However, the poison was switched by Vicki and instead, through the Doctor's intervention, and killed Tigilinus. For failing in this assignment Poppaea had Locusta taken away "to the arena". (DW: The Romans)