Disposing of one's rivals through the use of poisons was a widespread practice in ancient times but Locusta was a poisoner who would become history's female serial killer with the most victims.
She was born in the 1st century in what is today's France and showed a deep interest in herbs from an early age. Her hobby developed and she began looking for the poisonous parts of plants, creating various poisons from them.
When she moved to Rome, Locusta began developing a market for her poisons. Even though her activities were illegal there was actually a high interest. Dueling at that time was seen as uncivilized and the Romans looked for more secretive ways to get rid of their rivals.
Locusta was sought out frequently, though her fame was still in its early stages. Her fate would change when the wife of emperor Claudius - Agrippina the Younger - came to her, seeking a poison to use on her husband. Her goal was to make her son Nero the emperor but to do this she first had to get rid of Claudius.
Locusta gave her 2 poisons. She was to put one in the emperor's dinner, meant to cause severe pain but not actually kill. While feigning to help him, Agrippina was supposed to use the 2nd poison, hidden in a feather. And so she did.
Even after Nero took the throne, he did not forget the woman who had helped his mother. He himself wanted to kill his brother Britannicus but the 1st poison Locusta gave him did not achieve the desired effect, only making him bedridden.
It was time for plan B. Although Britannicus was very young he knew quite well that he shouldn't eat and drink everything he could get his hands on. Nero knew this as well and during dinner diluted his wine with hot water, a common practice in ancient Rome for heating up wine. Britannicus's wine taster tried the wine and determined it was safe.
However, Britannicus thought that the wine seemed too hot and decided to add cold water, in which Locusta's poison had been mixed ahead of time.
After the successful murders, Nero allowed Locusta to open her own school where she could teach poison making. As well, with the emperor's favor she was given the right to kill whomever she wanted.
Nero's reign would not last long however and after his death his professional poisoner was convicted. Some sources say she was beheaded, others that she was thrown to the lions.