Claudius died on the 13th October 54AD. This is the one fact about his death which can be proved by corroborating evidence, however when it comes to the manner in which he died or who was responsible for it the sources become less reliable.
Two ancient historians, Tacitus and Suetonius, both concur that Claudius was murdered by poison which was administered through an elaborate orchestration of Agrippina's. Tacitus and Suetonius also agree on Agrippina's motive for murdering Claudius and the prefect timing of it all too. It is their belief that Agrippina was a conniving and overly-ambitious woman who was determined that her son Nero be the next Emperor. They state that she had taken many steps to promote Nero over Claudius' own son Britannicus, even secured his adoption and preference as next to the throne as he was three years older than Britannicus. However it is believed that Agrippina and Nero were falling out of favour with Claudius towards the time of his death and Agrippina began to fear that he would renounce Nero as his son and promote Britannicus as the next Emperor instead.
Tacitus and Suetonius state that Agrippina chose to kill in 54AD in particular, as at this time Nero was almost of legal age to rule, but Britannicus was still three years too young, and she feared that if she left it any longer Britannicus would also be of legal age and would pose a threat to her and her son. This determination for Nero to be Emperor is evident in a quote from Suetonius where he states that 'Claudius' death was not revealed until all arrangements were made for Nero's succession.'
Despite the general consensus of why Agrippina poisoned Claudius, both Tacitus and Suetonius are somewhat unclear on how the poison was administered, and...