The last member of the Julio-Claudian line to rule the Roman principate was Lucius Domitius Ahenobarbus. He was the great grandson of Augustus through his daughter Julia and great great nephew through Augustus' sister Octavia. The son of Agrippina the younger (sister of Caligula) grew up in exile and poverty in the harsh circumstances of imperial intrigue; and his return to the forefront of the Roman imperial house was unlikely at best. However, the even more likely accession of Claudius allowed the return of exiled members of the Julio-Claudian house, and the eventual marriage between Claudius and Agrippina (uncle and niece) led to the adoption of Nero directly into the imperial line. Nero Claudius Caesar would eventually take precedence over Claudius' own son Britannicus through the scheming of his mother Agrippina, including the eventual marriage to Claudius' daughter Octavia. That scheming would also set the stage for Nero to rise as the next emperor unchallenged, as Agrippina methodically took control of the government and placed key supporters into positions of power.
The death of Claudius has been shrouded under the specter of murder (assumedly at the hands of Agrippina) though Claudius was 64 years old and of historically poor health. Still the systematic seizure of power by Agrippina, through her various political appointments, lends credence to the ancient source stories that Claudius was indeed poisoned. Regardless, in AD 54 Claudius was dead and the 17 year old Nero rose as the next Roman emperor. Under the tutelage of his mother, his tutor Seneca and the Praetorian Prefect Afranius Burrus, the first 5 years of Nero's reign was actually considered exemplary. Trajan later said that first five years of Nero's government were considered the happiest and best of the imperial era. Direct taxation was reduced as well as various restrictive governmental regulations. Capital punishment was outlawed and the games lessened while provincial administration continued to prosper as it had since the reign of Augustus.
Despite this quality start, it wouldn't take long for imperial intrigue to affect the order of things. A power struggle likely developed between Agrippina and Nero's advisors where, as an ambitious woman, she hoped to continue as a partner with Nero in imperial rule, and they resisted. Nero hated his wife Octavia, and began to have an affair with a former slave, Claudia Acte, which was supported by Seneca and Burrus. At this time too, Nero took less interest in the governing of the Empire but seemed more interested in the pursuance of the arts. singing, acting and playing the harp. indulgences that were considered fit for slaves. He also participated in nights of drunken revelry with friends, including random violence on the streets of Rome. Agrippina worried that through Nero's excesses and the growing strength of Seneca and Burrus (who privately supported Nero's antics in order for their own power to grow), that she would lose any grip on control of the Roman government. Not only did she support Octavia over Nero's various objections, but began to show favor to Claudius' son Britannicus as a potential replacement. This backfired however and Britannicus was murdered just shy of his 13th birthday in AD 55, likely though the intervention of Nero and Burrus, and Agrippina's influence would continue to decline.
Moving on from his slave concubine Acte, Nero was introduced to a more permanent relationship through his friend Otho (the future emperor, albeit short-lived). Otho was married to Poppaea Sabina and Nero was seemingly taken with her. Otto allowed a relationship to develop between the two and by AD 58 the affair was firmly in place. Agrippina continued to support Octavia (who as the daughter of Claudius helped establish Nero's legitimacy) and Poppaea assuredly complained over the interference. Nero finally made the decision to be rid of his pestering mother (this time without the support of Seneca and Burrus) and solicited the aid of the Prefect of the Fleet at Misenum, a freedman and former tutor by name of Anicetus. Rather than risk an open murder which would likely cause terrible shockwaves in a government that was slowly beginning to lose its popular appeal, Anicetus came up with an elaborate plan to have Agrippina brought to a party at Baiae via ship, which the fleet would provide. The ship however was constructed as collapsible so as to sink with the population being none the wiser at this tragic 'accident'. However, the plan failed and Agrippina was able to swim to safety and Nero was forced to adopt more direct methods. An assassin was sent to her villa and clubbed her to death, while a fabricated story of Agrippina's involvement in a plot against Nero would be circulated to justify the murder. In AD 59, one of the most powerful women in the history Roman society was dead, at the hand of her own son, after having been solely responsible for securing the position of ultimate power on his behalf.
Within a short time following the death of Agrippina, Nero's mistress Poppaea began to assert her own manner of control. Nero divorced Octavia, who was banished first to Campania. The public preferred Octavia though and their hatred of Nero and Poppaea would grow. Forced to be rid of Octavia so as not to be a constant threat, she was exiled to the island of Pandataria and beheaded shortly thereafter. By AD 62 Poppaea and Nero were married and political opponents began to disappear, opening the way for more administration officials of dubious quality. Burrus had died in that same year to be replaced by a supporter of Poppaea, Sophonius Tigellinus, and Seneca was forced into retirement under this new regime. Nero continued to riot through the city with his drunken friends, beating strangers and assaulting women. Without Seneca, Burrus and even Agrippina to dictate some form of controlled or effective government, the reign of Nero devolved rapidly. Extravagance and luxurious spending grew and the treasury was in serious jeopardy. Still, however, the worst was yet to come.