When Messalina married Gaius Silius in 48, it was Narcissus who betrayed her to Claudius, and seeing the emperor hesitate, he gave the order for her execution himself. Narcissus may have feared that Claudius' son, Britannicus would hold a grudge against him for this role. When the time came for the emperor to select a new wife, Narcissus suggested Aelia Paetina, Claudius' ex-wife - whom he would have known. Anthony Barrett suggests that Narcissus' intention was to allow Claudius reason to pick Faustus Cornelius Sulla Felix, the husband of his and Aelia's daughter Claudia Antonia, as his successor rather than a hostile Britannicus. It would also have given Claudius an adult heir, which he was looking for to shore up his position. When Claudius chose Agrippina the Younger in order to consolidate the Julio-Claudian family, and picked Nero to fill the role of temporary older heir, Narcissus allied with Britannicus' circle in order to secure his future.
Claudius still trusted Narcissus, and had him named praetor. He was charged with overseeing the building of a canal to drain Lacus Fucinus, but Claudius's new wife Agrippina accused him of embezzling funds from the project, possibly as punishment for his support of Britannicus. According to Tacitus, Narcissus hoped to bring down Agrippina by revealing her affair with the freedman Pallas, which would also have destroyed her son. He supposedly told Britannicus of his plans in front of others, and was brazen in his intentions, promising to right all wrongs against him. It has been suggested that this last detail is an example of Tacitus altering facts to make Claudius a passive character in his reign. Suetonius and Dio report that, after reconciling with Brittanicus, Claudius -- not Narcissus -- openly planned to bring Agrippina down. In any case, Agrippina was suspicious of Narcissus and had him sent away to Campania, ostensibly to take advantage of the warm baths there to relieve his gout. This was probably intended to remove him as an obstacle to the assassination of Claudius and the accession of Nero. Agrippina ordered Narcissus' execution within weeks of Claudius' death in October, 54. Shortly after the announcement Narcissus returned to Rome. Before his imprisonment and execution, he burned all Claudius' letters to prevent Nero from using their contents for nefarious means.
Narcissus is a character in the Apocolocyntosis of Seneca the Younger, written soon after his death. He greets Claudius in Hades and runs ahead of him through the gates of the underworld. He is scared by Cerberus, a dog-beast so unlike the little white dog Narcissus is mentioned as owning in life. An inscription names his wife as Claudia Dicaeosyna.