Locusta

Locusta was a professional poisoner in Rome, Vicki met her whilst she and the 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) poisoned and killed Tigilinus. For failing in this assignment Poppaea had Locusta taken away "to the arena". (DW: The Romans)