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.
Insurrection of Maternus
[1.10.1]  But before long another plot was organized against Commodus. It involved a former soldier named Maternus, who had committed many frightful crimes.note He deserted from the army, persuading others to flee with him, and soon collected a huge mob of desperadoes. At first they attacked and plundered villages and farms, but when Maternus had amassed a sizable sum of money, he gathered an even larger band of cutthroats by offering the prospect of generous booty and a fair share of the loot. As a result, his men no longer appeared to be brigands but rather enemy troops.
[1.10.2] They now attacked the largest cities and released all the prisoners, no matter what the reasons for their imprisonment. By promising these men their freedom, he persuaded them to join his band in gratitude for favors received. The bandits roamed over all Gaul and Spain, attacking the largest cities; a few of these they burned, but the rest they abandoned after sacking them.
[1.10.3] When he was informed of these developments, Commodus, in a towering rage, sent threatening dispatches to the governors of the provinces involved, charging them with negligence and ordering them to raise an army to oppose the bandits. When the brigands learned that an army was being raised against them, they left the regions which they had been ravaging and slipped unnoticed, a few at a time, into Italy, by a quick but difficult route. And now Maternus was plotting for the empire, for larger stakes indeed. Since everything he had attempted had succeeded beyond his fondest hopes, he concluded that if he were to undertake something really important it was bound to succeed; having committed himself to a hazard from which it was impossible to withdraw, he would, at least, not die obscure and unknown.
[1.10.4] But when he reflected that he did not have an army sufficiently powerful to resist Commodus on equal terms and in open opposition (for it was thought that the majority of the Roman people were still well disposed toward Commodus, and he also had the support of the Praetorian Guard), Maternus hoped to balance this inequality of forces by guile and cunning. This is the way he undertook to accomplish it.
[1.10.5] Every year, on a set day at the beginning of spring, the Romans celebrate a festival in honor of the mother of the gods [Cybele]. All the valuable trappings of each deity, the imperial treasures, and marvelous objects of all kinds, both natural and man-made, are carried in procession before this goddess. Free license for every kind of revelry is granted, and each man assumes the disguise of his choice. No office is so important or so sacrosanct that permission is refused anyone to put on its distinctive uniform and join in the revelry, concealing his true identity; consequently, it is not easy to distinguish the true from the false.
[1.10.6] This seemed to Maternus an ideal time to launch his plot undetected. [March 187] By donning the uniform of a praetorian soldier and outfitting his companions in the same way, he hoped to mingle with the true praetorians and, after watching part of the parade, to attack Commodus and kill him while no one was on guard.
[1.10.7] But the plan was betrayed when some of those who had accompanied him into the city revealed the plot. (Jealousy led them to disclose it, since they preferred to be ruled by the emperor rather than by a bandit chief.) Before he arrived at the scene of the festivities, Maternus was seized and beheaded, and his companions suffered the punishment they deserved. After sacrificing to the goddess and making thank offerings, Commodus completed the festivities and did honor to the goddess, rejoicing at his escape. The people continued to celebrate their emperor's deliverance after the festival came to an end.