Locusta was a real-life Roman poisoner who appeared in the Doctor Who serial "The Romans", portrayed by actress Anne Tirard.
While at the court of Emperor Nero, a time traveller named Vicki Pallister befriended Locusta, who told her that she was in constant demand because there was always some kind of intrigue going on at the palace. She also hinted that she wanted to one day poison Nero. However, their conversation was interrupted by Poppaea Sabina, Nero's wife, who told Locusta that she wanted her to poison a slave who she believed was having an affair with Nero. Locusta agrees, but after they both left the room Vicki switched the cups so Nero would be poisoned instead. However, another time traveller called the Doctor stopped Nero from drinking the wine and Poppaea, furious that she failed, had Locusta arrested.