Herodian (c.170 - c.240): 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).
Marcus' final years
[1.3.1]  When Marcus was an old man, exhausted not only by age but also by labors and cares, he suffered a serious illness while visiting the Pannonians. When the emperor suspected that there was little hope of his recovery, and realized that his son would become emperor while still very young, he was afraid that the undisciplined youth, deprived of parental advice, might neglect his excellent studies and good habits and turn to drinking and debauchery (for the minds of the young, prone to pleasures, are turned very easily from the virtues of education) when he had absolute and unrestrained power.
[1.3.2] This learned man was disturbed also by the memory of those who had become sole rulers in their youth. The Sicilian despot Dionysus, in his excessive licentiousness, had sought out new pleasures and paid the highest prices for them. The arrogance and violence of Alexander's successors against their subject peoples had brought disgrace upon his empire.
[1.3.3] Ptolemy [II Philadelphus], too, contrary to the laws of the Macedonians and Greeks, went so far as to marry his own sister [Arsinoe II]. Antigonus [Monophthalmus] had imitated Dionysus in every way, even wearing a crown of ivy instead of the Macedonian hat or the diadem, and carrying the thyrsus instead of a scepter.
[1.3.4] Marcus was even more distressed when he recalled events of recent date. Nero had capped his crimes by murdering his mother and had made himself ridiculous in the eyes of the people. The exploits of Domitian, as well, were marked by excessive savagery.
[1.3.5] When he recalled such spectacles of despotism as these, he was apprehensive and anticipated evil events. Then, too, the Germans on the border gave him much cause for anxiety. He had not yet forced all these tribes to submit; some he had won to an alliance by persuasion; others he had conquered by force of arms. There were some who, although they had broken their pact with him, had returned to the alliance temporarily because of the fear occasioned by the presence of so great an emperor. He suspected that, contemptuous of his son's youth, they would launch an assault upon him; for the barbarian is ever eager to revolt on any pretext.