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.
[3.11.1]  Observing that Severus was now old and constantly racked by disease,note[Severus suffered from gout. The Senate twice offered him a triumphal entry into the city, but because he could not drive a chariot, he refused.] while Caracalla was a rash and reckless youth, Plautianus, in fear of these threats, elected to act first rather than to delay and suffer at his son-in-law's hands.
[3.11.2] Moreover, a number of things encouraged him to aspire to control of the empire: more money than anyone had ever seen before, his own personal army, honors from the emperor, and his public attire. He wore the toga with the [senatorial] purple border and held rank equal to that of men who had twice served as consul; also he wore a sword. The prefect was the only official whose appearance suggested his importance.
[3.11.3] He was an object of dread when he appeared in public; not only did no one approach him, but even those who came upon him by chance turned aside to avoid him. The guards who preceded him did not allow anyone to stand near or to stare at him; all were ordered to step aside and keep their eyes fixed on the ground. Severus was far from pleased when these matters were brought to his attention; he now became stern and harsh with Plautianus and tried to curb his excessive ostentation by depriving him of some of his authority.
[3.11.4] Plautianus refused to tolerate this reduction of power; bold enough to plot for the empire, he devised the following plan. In the Praetorian Guard was a tribune named Saturninus. This officer was devoted to the prefect; in fact, all his officers were devoted to Plautianus, but he had won the favor of the tribune by treating him with greater affection. Believing that Saturninus was the most trustworthy of the praetorian officers and the only one capable of using discretion and of carrying out secret orders, Plautianus summoned the man to him one evening after the rest had gone to bed.
[3.11.5] "Now you have an opportunity," he said, "to bring to a proper climax the good will and devotion you have always shown me, and I equally have an opportunity to repay you as you deserve and do you a comparable favor in return. The choice is yours either to become what you now see me to be and to secure this office of authority by succeeding me, or to die here and now, paying the penalty for disobedience.
[3.11.6] Do not by any means be overawed by the enormity of the deed I propose, and do not be disturbed by the title of emperor. You are the only one who can go into the room where the emperor and his son are sleeping, since you are in charge of the regular rounds of the night unit of the guard. Whatever you intend to do, you will do secretly and without interference; do not wait for me to issue the orders before you obey them.
[3.11.7] Go immediately to the imperial palace and, pretending to be carrying secret orders from me, go in and kill them. Show your courage by dispatching with ease an old man and a mere boy. And for sharing the risk and the danger, you will also share the highest honors when the deed is successfully done."
[3.11.8] The tribune was astounded and perplexed by this proposal, but he was a man accustomed to keeping his wits about him (he was a Syrian, and the men from the East are rather more cunning in their thinking); observing the fury which gripped his commanding officer and well aware of his power, he did not oppose him, not wishing to be killed over these matters. Pretending therefore to be hearing things long prayed for and warmly welcomed, the tribune prostrated himself before Plautianus as if he were already emperor and begged him for a written memorandum ordering the murder.
[3.11.9] If a man were condemned to death without a trial, the tyrants customarily put the order in writing so that the sentence might not be carried out solely on verbal authority. Blinded by his ambition, Plautianus gave the tribune a directive in writing and sent him off to commit the murders. He further ordered Saturninus, after killing the emperor and his son, to summon him, before the deed became known, that he might be in the palace before anyone realized that he was seizing the empire.