Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
Death of Severus
[3.15.1]  Now a more serious illness attacked the aged emperor and forced him to remain in his quarters; he undertook, however, to send his son out to direct the campaign. Caracalla, however, paid little attention to the war, but rather attempted to gain control of the army. Trying to persuade the soldiers to look to him alone for orders, he courted sole rule in every possible way, including slanderous attacks upon his brother.
[3.15.2] Considering his father, who had been ill for a long time and slow to die, a burdensome nuisance, he tried to persuade the physicians to harm the old man in their treatments so that he would be rid of him more quickly.note[4 February 217.] After a short time, however, Severus died, succumbing chiefly to grief, after having achieved greater glory in military affairs than any of the emperors who had preceded him.
[3.15.3] No emperor before Severus won such outstanding victories either in civil wars against political rivals or in foreign wars against barbarians. Thus Severus died after ruling for eighteen years, and was succeeded by his young sons, to whom he left an invincible army and more money than any emperor had ever left to his successors.
[3.15.4] After his father's death, Caracalla seized control and immediately began to murder everyone in the court; he killed the physicians who had refused to obey his orders to hasten the old man's death and also murdered those men who had reared his brother and himself because they persisted in urging him to live at peace with Geta. He did not spare any of the men who had attended his father or were held in esteem by him.
[3.15.5] He undertook secretly to bribe the troop commanders by gifts and lavish promises, to induce them to persuade the army to accept him as sole emperor, and he tried every trick he knew against his brother. He failed to win the backing of the soldiers, however, for they remembered Severus and knew that the youths had been one and the same to him, and had been reared as equals from childhood; consequently they gave each brother the same support and loyalty.
[3.15.6] When the soldiers refused to uphold him, Caracalla signed a treaty with the barbarians, offering them peace and accepting their pledges of good faith. And now he abandoned this alien land and returned to his brother and mother. When the boys were together again, their mother tried to reconcile them, as did also men of repute and the friends of Severus who were their advisers.
[3.15.7] Since all these opposed his wishes, Caracalla, from necessity, not from choice, agreed to live with Geta in peace and friendship, but this was pretended, not sincere. Thus, with both of them managing imperial affairs with equal authority, the two youths prepared to sail from Britain and take their father's remains to Rome. After burning his body and putting the ashes, together with perfumes, into an alabaster urn, they accompanied this urn to Rome and placed it in the sacred mausoleum of the emperors.
[3.15.8] They now crossed the Channel with the army and landed as conquerors on the opposite shore of Gaul. How Severus came to the end of his life and how his sons succeeded him in the imperial power, I have described in this book.