"His rage was so violent and so unbounded that he burst an artery." ~ Valentinian I dies of apoplexy in AD 375

A gold solidus of Valentinian I.

On November 17 (according to Jones in The Later Roman Empire) or November 18 (according to the Loeb translation of Ammianus Marcellinus's Roman History), the Roman emperor Valentinian I died in a most dramatic and unexpected way.

Valentinian rose to the throne in AD 364 following the death of Jovian. The story of his accession may be found in a previous post here. His twelve year reign was marked by numerous barbarian incursions which Valentinian, for the most part, was able to contain and defeat. To a certain extent, he was the last emperor to be fully in control of the situation in the West. He fought numerous campaigns to defend the frontiers and defeat invasions. He also spent considerable time conducting punitive raids across the Rhine and Danube, while sending his capable officer, Theodosius (the father of Theodosius the Great), to Britain to defeat an incursion of Picts that had thrown the province into chaos.

When an large invasion of Quadi and Sarmatians poured across the frontier in AD 374 to devastate Pannonia and Moesia, Valentinian mobilized his forces to oppose them. What happened next is recorded by Hermias Sozomen in his Ecclesiastical History: 

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The Sarmatians having invaded the Western provinces of the empire, Valentinian levied an army to oppose them. As soon, however, as they heard of the number and strength of the troops raised against them, they sent an embassy to solicit peace. When the ambassadors were ushered into the presence of Valentinian, he asked them whether all the Sarmatians were similar to them. On their replying that the principal men of the nation had been selected to form the embassy, the emperor exclaimed in great fury, that he regarded it as an especial misfortune that the territories under his sway should be exposed to the incursions of a barbarous nation like the Sarmatians, who had even presumed to take up arms against the Romans! He spoke in this strain for some time in a very high pitch of voice, and his rage was so violent and so unbounded, that at length he burst simultaneously a blood-vessel and an artery. He lost, in consequence, a great quantity of blood, and expired soon after in a fortress of Gaul.

He was about fifty-four years of age and had during thirteen years guided the reins of government with great wisdom and skill. [Ecclesiastical History of Sozomen, Book VI, Chapter XXXVI]

Ammianus Marcellinus adds some vivid detail to the scene of Valentinian's death:

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Then he gradually calmed himself and seemed more inclined to mildness, when, as if struck by a bolt from the sky, he was seen to be speechless and suffocating, and his face was tinged with a fiery flush. On a sudden his blood was checked and the sweat of death broke out upon him. Then, that he might not fall before the eyes of a throng of the common sort, his body-servants rushed to him and led him into an inner chamber. There he was laid upon a bed; but although he was drawing more feeble remnants of breath, the vigor of his mind was not yet lessened, and he recognized all those who stood about him, whom the chamberlains had summoned with all speed, in order to avert any suspicion that he had been murdered. And since all parts of his body were burning hot, it was necessary to open a vein, but no physician could be found, since he had sent them to various places, to give attention to the soldiers who were attacked by the plague. At last however one was found, but although he repeatedly pierced a vein, he could not draw even a single drop of blood, since the emperor's inner parts were consumed by excessive heat, or (as some thought) because his body was dried up, since some passages for the blood (which we now call haemorrhoidae) were closed and incrusted by the cold chills. He felt the disease crushing him with a mighty force, and knew that the fated end of his life was at hand; and he tried to speak or give some orders, as was indicated by the gasps that often heaved his sides, by the grinding of his teeth, and by movements of his arms as if of men fighting with the cestus; but finally his strength failed him, his body was covered with livid spots, and after a long struggle for life he breathed his last... [Roman History of Marcellinus, Book XXX, Chapter 6].

Sozomen summarizes what happened next:

Six days after his death, his youngest son who bore the same name as himself, was proclaimed emperor by the soldiers, and soon afterwards Valens and Gratian formally assented to this election, although they were at first irritated at the soldiers having adopted such a measure without their sanction.

For more details on the succession, as well as a long appraisal of Valentinian's character, good points and bad, see the chapters in Ammianus following the account of his death.