Locusta was a professional poisoner in Rome.
In July 64, Vicki Pallister met Locusta whilst she and the First Doctor, who was impersonating Maximus Pettulian, were in 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 given to Nero. Nero was spared death through The Doctor’s intervention. Nero gave the poisoned cup to Tigilinus, who subsequently died. For failing in this assignment, Poppaea had Locusta taken away “to the arena”.