Julia Agrippina

After discovering Messalina's Crimes, Claudius told his soldiers to kill him if he was such a fool as to marry again, but he did anyhow. This time, it seemed he had found the one woman in Rome worse than Messalina, his niece Agrippina.
She was the daughter of Claudius' brother, the great general Germanicus, and sister of the emperor Caligula. She had been married to Gnaeus Domitius Ahenobarbus (Bronzebeard) of whom the Roman historian Suetonis calls a "wholly despicable character." Agrippina's son by him, Domitius would eventually become Emperor Nero.
When Caligula became emperor, he decided he was a god and imitated the domestic lives of the gods by sleeping with his three sisters, Drusilla, Livilla and Agrippina. He also setup a brothel in the palace where he sold his sisters and charged high rates. Ultimately he got tired of having them around the court and exiled Livilla and Agrippina to a small island where they were forced to earn their bread by diving for sponges. Claudius, who was kind hearted, allowed the girls to return to Rome when he took the throne.
After Messalina's assassination Agrippina saw an opportunity to win Claudius' affections and soon convinced Pallas, Claudius' financial secretary and her lover, to push her as a candidate for empress. Soon enough Claudius grew quite fond of her and began to understand the many reasons why a marriage between them would make sense: she was intelligent and cunning and could help with the work of governing; her son was a worthy member of the imperial family; she might give the emperor an heir and as a rich member of the imperial family, it would be unsafe to let her marry anyone else who might get ambitious for the imperial throne. The Roman Senate, well trained in servility by its dealings with Tiberius and Caligula agreed to the marriage unanimously in spite of the fact that incestuous marriages were forbidden by Roman law. In 49, she married her third husband and uncle Tiberius Claudius Caesar after the Senate removed the prohibition against the legal marriage between uncle and niece.
By this time, Claudius was venerable and nearing the end of his life. Agrippina, being an ambitious and intelligent woman married to an emperor considered a weakling and somewhat of a dunce by those around him, naturally took the reins of power into her own hands and quickly became the main power of government. Tacitus says, "Unlike Messalina, she did not dabble in politics for fun. She brought to it an almost masculine sense of service; her seriousness was obvious, as was, often, her arrogance."
Agrippina persuaded Claudius to adopt Nero as his son and to arrange a marriage between marry his daughter Octavia and Nero. She prepared to make him, not Claudius' son Britannicus, the heir to the throne. She also guarded her own position by having murdered women whose beauty Claudius had praised. Officials whose loyalty was to Claudius were replaced by her own men. When she decided it was time to remove Claudius, poison was her chosen method. She gave Claudius a particularly choice mushroom from a dish she was eating: it was the only poisoned one. Claudius was given a fine funeral and proclaimed a god. Nero, in later years used to laugh by referring to mushrooms as "the food of the gods."
When Nero became emperor he was only seventeen, and could not legally rule so for the first year of his reign Agrippina basically ran the empire until he was of legal age. She was the first woman in the history of Rome to be given the title of empress. Her picture later appeared on coins with her son; she was the first woman of the imperial household to be pictured on coins while she was still alive. Her son, who appreciated what his mother had done for him lovingly called her "the best of mothers" but eventually he began to resent her power and control. Agrippina wanted an open share in government which the Romans thought was scandalous. She thwarted Nero's love affairs, forbid him to do as he chose and often threatened him that she would expose his treatment of her and make Britannicus emperor in his place. Nero ended this threat by poisoning 14 year old Britannicus at the imperial table. He cunningly bypassed the boy's taster appointed to protect him by handing him a cup in which the liquid was too hot, after having tasted it, a servant poured in poisoned cold water in the drink which was then handed to the boy. After one sip Britannicus went into convulsions and died instantly. Nero "lay back unconcernedly," claiming that Britannicus was merely having an epileptic fit. Octavia, Britannicus' sister, though young, "had learnt to hide sorrow, affection, every feeling. After a short silence the banquet continued."
Nero exiled Agrippina from the palace. Soon enough hired informers accused her of plotting Nero's death, but Agrippina faced down the charges declaring they were false. Nero had at this time fallen in love with Sabina Poppae (who desperately wanted to be empress). Agrippina fiercely opposed this union and the two women fought for control of the emperor. Agrippina went as far as to sleep with her son to retain her influence, but Nero knew the only way to be rid of her was to kill her. Avoiding poison, which would remind people of the fate of Claudius and Britannicus, he tried several ingenious devices. All failed. Finally he planned a shipwreck . He planned a ship that would collapse at sea then gave a banquet of reconciliation for his mother. He treated her with great respect, then ushered her to her ship and bade her farewell, kissing her affectionately on the eyes and bosom. Undoubtedly, at this point, Nero thought his problems were over.
When the planned accident occurred, Agrippina's two companions were killed. She was wounded, but swam to shore anyhow. Her past as a sponge diver proved handy. She sent a messenger to inform her son of her escape pretending to have no suspicions of his guilt so that she would have time to plan her revenge. The terrified Nero, knowing that his mother's revenge was sure to be deadly, framed the massager by accusing him of attacking the emperor and then sent a freedman to kill his mother. Her last words were supposed to have been, "Strike here," as she pointed to the womb from which Nero was borne.
Agrippina's alleged victims:
Passienus, her second husband, poisoned
Calpurnia, distinuished woman admired by Claudius, banished and later executed.
Lollia Paulina, ex-wife of Caligula, candidate for marriage to Claudius, executed
Titus Statilius Taurus, consul in 44 AD, forced into suicide
Domitia Lepida, mother of Messalina and sister-in-law of Agrippina, executed
Claudius, her husband, poisoned
Marcus Junius Silanus, potential rival to Nero, poisoned
Junia Silana, enemy of Agrippina, exiled
Calvisius and Iturius, clients of Junia Silana, exiled
Atimetus, freedman of Domitia Lepida, executed