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.
Conspiracy of Lucilla
[1.8.1] For several years the emperor deferred to the advisers appointed by his father, following their advice in everything.  But when he assumed absolute control of the empire, he put in command of the Praetorian Guard an Italian, Perennis, who seemed to be a capable soldier. (Indeed, it was for this reason that Commodus made him praetorian prefect.) Perennis indulged the emperor's youthful appetites, permitting him to spend his time in drinking and debauchery, and relieved him of imperial cares and responsibilities.
[1.8.2] Perennis assumed full personal charge of the empire, driven by his insatiable lust for money, his contempt for what he had, and his greedy longing for what was not yet his. To begin with, he launched an attack upon Commodus' advisers and upon all the wealthy and nobly born; by casting suspicion upon these men, Perennis aroused the fears of the emperor and provided the youth with reason and opportunity to destroy them and confiscate their property.
[1.8.3] For the present, however, the memory of his father and his respect for his advisers held Commodus in check. But then a disastrous stroke of ill fortune completely altered his previously mild, moderate disposition. It happened this way. The oldest of the emperor's sisters was Lucilla. She had formerly been married to Lucius Verus Caesar, whom Marcus had made his associate in governing the empire; by marrying Lucilla to Lucius, Marcus had made her marriage to his Caesar the strongest bond of mutual good will. But after Lucius died, Lucilla, who retained all the privileges of her imperial position, was married by her father to Pompeianus.
[1.8.4] Commodus, too, allowed his sister to retain the imperial honors; she continued to occupy the imperial seat at the theaters, and the sacred fire was carried before her. But when Commodus married Crispina,note[Although Herodian suggests that Commodus and Crispina married after his accession in 180, this in fact happened in 178. Whatever Lucilla's motive, it can not have been that Commodus had insulted her. She was sent into exile to Capri, where she was later executed.] custom demanded that the front seat at the theater be assigned to the empress. Lucilla found this difficult to endure, and felt that any honor paid to the empress was an insult to her; but since she was well aware that her husband Pompeianus was devoted to Commodus, she told him nothing about her plans to seize control of the empire. Instead, she tested the sentiments of a wealthy young nobleman, Quadratus, with whom she was rumored to be sleeping in secret. Complaining constantly about this matter of imperial precedence, she soon persuaded the young man to set in motion a plot which brought destruction upon himself and the entire Senate.
[1.8.5]  Quadratus, in selecting confederates among the prominent senators, prevailed upon Quintianus, a bold and reckless young senator, to conceal a dagger beneath his robe and, watching for a suitable time and place, to stab Commodus; as for the rest, he assured Quintianus that he would set matters straight by bribes.
[1.8.6] But the assassin, standing in the entrance to the amphitheaternote[The amphitheater of Rome is better known as Colosseum.] (it was dark there and he hoped to escape detection), drew his dagger and shouted at Commodus that he had been sent by the Senate to kill him. Quintianus wasted time making his little speech and waving his dagger; as a result, he was seized by the emperor's bodyguards before he could strike, and died for his stupidity in revealing the plot prematurely. Thus found out beforehand, Quintianus brought about his own death, and Commodus was put on his guard by this forewarning.
[1.8.7] This was the initial reason for the young emperor's hatred of the Senate. He took Quintianus' words to heart and, ever mindful of what his attacker had said, now considered the entire Senate his collective enemy.
[1.8.8] This incident also gave Perennis sufficient excuse for taking action, for he was always advising the emperor to eliminate and destroy the prominent men. By confiscating their property, Perennis easily made himself the richest man of his time. After the attempt at assassination had been thoroughly investigated by the prefect, Commodus without mercy put to death his sister, all those actually involved in the plot, and any who were under the slightest suspicion as well.