By AD 38, and prior to his rise to Emperor, Claudius was married (for the third time) to the 15 year old Valeria Messalina. The young 'empress' was portrayed historically as not much more than a court nymphomaniac who used her sexual prowess to influence the influential. She did, however give Claudius two children: Octavia (AD 39) and Britannicus (AD 41). Though stories of wild parties, intrigue and murder follow Messalina from the ancient sources, some modern scholars have painted her as an astute player in the political world of the time. Either way, she was well known for various sexual escapades, regardless of her own motivation, and other scheming while the stuttering Claudius was either completely unaware of refused to see it. She used her power to favor friends and punish her enemies (not all that unusual really) and had Claudius banish, and eventually execute his niece Julia (the sister of Caligula who had already been recalled once before) for adultery with L. Annaeus Seneca. Seneca, the author and influential politician was also exiled to Corsica for a time and would later have his revenge by publishing the scathing satirical attack on Claudius: Apocolocyntosis Divi Claudii (Pumpkinification of the Divine Claudius).
After 10 years of marriage the plot thickened beyond promiscuity, however. What may have been a plotted coup attempt came to fruition while Claudius was away in Ostia c. AD 48. Messalina declared herself divorced from Claudius and was married in a somewhat private ceremony to a designated Consul for the following year, C. Silius. Although the wedding arrangements had been made privately, the wild party that followed the ceremony helped give away the secret. Certainly Silius would've been aware of the danger in such a move, considering he was marrying the emperor's wife and his own influential (read as dangerous) status as a descendent of an Augustan general. Perhaps the idea was to replace Claudius and act as regents until the youthful Britannicus could come of age. Regardless, whether it was an actual coup attempt or a semi secret love affair, word reached Claudius (largely through his powerful freedmen) and after some consternation Messalina was eventually put to death. A large number of 'conspirators' joined Messalina's fate lending support to the idea that at least Claudius viewed it as a coup attempt whether it was in reality or not.
With the death of Messalina, Claudius' freedmen vied for supreme influence over the emperor by supporting various marriage prospects as replacements. In the end, the freedman Pallas won the competition but also assigned Claudius and the empire to a terrible twist of fate. The candidate who won was his own niece, Agrippina (the Younger), sister of Caligula. Likely having little to do with anything other than political connotations (she was the great granddaughter of Augustus) the marriage between uncle and niece (AD 49) required a change in the law. She had been previously married to Cn. Domitius Ahenobarbus, a powerful Republican family in its own right, and came to Claudius with a son Lucius Domitius Ahenobarbus (Nero). This is significant in that Agrippina was highly motivated regarding the advancement of her son and would exert great influence over Claudius and the government of Rome in order to achieve her ends.
Ahenobarbus was 4 years older than Claudius' natural son Britannicus and Agrippina convinced Claudius that adopting her son was better for the preservation of the principate. Thus, Ahenobarbus became Nero Claudius Drusus Germanicus Caesar and would eventually be advanced over Britannicus as Claudius' heir. To further solidify that position, Agrippina had the fiance of Claudius daughter prosecuted in order to make her available to marry Nero. A potential rival marriage candidate for Claudius, Lollia Paulina, was driven to suicide, and Agrippina would exert such authority as to be declared an official living Empress (or Augusta). Only Livia (wife of Augustus) had been so before, and only after her death. According to the ancient sources, between her rise and Claudius' death in 54 AD, Agrippina systematically took control of the Imperial government while Claudius was left to appear as a figurehead in front of his wife's real power. She wore a military cloak at official state functions, greeted foreign embassies in the capacity of full imperial authority, appeared prominently on coinage and had her dictations recorded in official government documentation.
Perhaps most importantly, Agrippina used her influence to surround and protect her position with men loyal to her and her son. Seneca, who despised Claudius but kept it in check, was recalled and installed as Nero's tutor. Back in Rome he used his brilliant political skill to influence the imperial court to Agrippina's wishes. Additionally, her own choice of Sextus Afranius Burrus was appointed as the all important Praetorian Prefect and as a second tutor for Nero, with the obvious ramifications that suggests. Meanwhile the young Nero continued to be advanced as the heir to Claudius while Britannicus languished behind, virtually invisible compared to Nero. He was granted full imperial authority outside the city (where Claudius retained singular control in theory) addressed the Senate, appeared with Claudius at the games (obvious indication of his role as heir) and was inscribed as such on imperial coinage.
By 54 AD, according to the ancients, Agrippina was secured enough in her position, and that of her son, that she no longer needed Claudius to rule the empire. Tacitus suggests that Claudius resisted the final steps to secure Nero as heir, and Agrippina, rather than wait him out, decided to take matters into her own hands. On October 13, AD 54, Claudius died while attending a feast. Though the reports are conflicting all indicate that he was poisoned by tainted mushrooms, even though Claudius had reached the venerable age of 64 (quite advanced for the ancient world, though not uncommon among the aristocracy) and had shown a history of poor health. Regardless, the scheming of Agrippina proved fruitful and the 16 year old Nero was immediately hailed as the new Emperor without any consideration for the much younger Britannicus.
Claudius was quickly deified without resistance, despite his poor relationship with the Senate, though his Imperial cult received little attention under Nero's reign. Claudius' reign is subject to much debate of course. Was he the pitiful, bumbling, murderous and spiteful fool described by the ancients, or the highly intelligent, excellent administrator, yet susceptible dupe to his advisors and wives as portrayed by Robert Graves in his well regarded novels? The truth probably lies somewhere in between. Claudius advanced the Empire through the conquest of Britain and made citizenship more inclusive through provincial enrollment. He built great public works and generally kept the peace, certainly earning at least the respect, if not the admiration of the people. Claudius is either remembered as this quality fourth member of the Julio-Claudian line who stabilized the principate after the death of Caligula, or the cruel bumbling butcher who also left the world with the incompetent and disastrous Nero as the heir to the throne.