Tacitus describes the suicide of Seneca in his Annals.
Seneca was an eminent writers of Silver Age Latin. Born Lucius Annaeus Seneca, in Córdoba Spain, he was the son of the Roman rhetorician Marcus (Lucius) Annaeus Seneca, known as Seneca the Elder.
Seneca ran afoul of the Julio-Claudian emperors, ultimately, fatally, but first, Caligula was dissuaded from killing him and Claudius merely had him exiled to Corsica. However, after a falling out with Nero and a charge of being involved in the conspiracy of Piso, Seneca's luck ran out. Seneca took his own life, as befitted a Stoic philosopher and Roman, in a lingering, pathetic manner, in 65 A.D., detailed by Tacitus
"Seneca, as his aged frame, attenuated by frugal diet, allowed the blood to escape but slowly, severed also the veins of his legs and knees.
"Seneca meantime, as the tedious process of death still lingered on, begged Statius Annaeus, whom he had long esteemed for his faithful friendship and medical skill, to produce a poison with which he had some time before provided himself, same drug which extinguished the life of those who were condemned by a public sentence of the people of Athens. It was brought to him and he drank it in vain, chilled as he was throughout his limbs, and his frame closed against the efficacy of the poison. At last he entered a pool of heated water, from which he sprinkled the nearest of his slaves, adding the exclamation, "I offer this liquid as a libation to Jupiter the Deliverer." He was then carried into a bath, with the steam of which he was suffocated, and he was burnt without any of the usual funeral rites."
Ancient History Sourcebook: Tacitus: The Death of Seneca 65 CE
Conspiracy of Piso From Suetonius Life of NeroXXXVI....[Nero] felt great anxiety on account of this phenomenon, and being informed by one Babilus, an astrologer, that princes were used to expiate such omens by the sacrifice of illustrious persons, and so avert the danger foreboded to their own persons, by bringing it on the heads of their chief men, he resolved on the destruction of the principal nobility in Rome. He was the more encouraged to this, because he had some plausible pretence for carrying it into execution, from the discovery of two conspiracies against him; the former and more dangerous of which was that formed by Piso , and discovered at Rome.... The conspirators were brought to their trials loaded with triple fetters. Some ingenuously confessed the charge; others avowed that they thought the design against his life an act of favour for which he was obliged to them, as it was impossible in any other way than by death to relieve a person rendered infamous by crimes of the greatest enormity. The children of those who had been condemned, were banished the city, and afterwards either poisoned or starved to death....
Suetonius Life of Nero