Tiberius Claudius Caesar Britannicus, usually called Britannicus, was the son of Roman emperor Claudius and his third wife Valeria Messalina. For a time he was considered his father's heir, but that changed after his mother's downfall in 48, when it was revealed she had engaged in a bigamous marriage without Claudius' knowledge. The next year, his father married Agrippina the Younger, Claudius' fourth and final marriage. Their marriage was followed by the adoption of Agrippina's son, Lucius Domitius, whose name became Nero as a result. His step-brother would later be married to Britannicus' sister Octavia, and soon eclipsed him as Claudius' heir. Following his father's death in October 54, Nero became emperor. The sudden death of Britannicus shortly before his fourteenth birthday is reported by all extant sources as being the result of poisoning on Nero's orders—as Claudius' natural son, he represented a threat to Nero's claim to the throne.