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.
Severus and Albinus
[3.5.1]  After settling Eastern affairs in what he thought was the most advantageous way,note[Syria was divided into two provinces, Coele Syria and Phoenicia. Herodian ignores Severus' first attack on Parthia, which took place in 194/195 and resulted in the occupation of Edessa and Nisibis. Our historian also ignores that Severus' son Caracalla was made caesar, which explains why Severus and Albinus started to quarrel.] Severus wished to take the field immediately against the Hatrenian king and invade Parthia also, charging both of these kings with friendship for Niger. He put off these projects until later, however, wishing to seize the Roman empire first and make it secure for himself and his sons.
[3.5.2] Even though Niger had been eliminated, Severus considered Albinus still a menace. He now heard that this man, delighted with the title of caesar, was acting more and more like an emperor; he was informed also that a great many men, particularly the most distinguished senators, were writing public and private letters to the caesar, trying to persuade him to come to Rome while Severus was absent and occupied elsewhere. The fact is that the aristocracy much preferred Albinus as emperor because he belonged to a noble family and was reputed to have a mild nature.
[3.5.3] When he learned of these developments, Severus declined to initiate open hostility against Albinus and start a war with him since he lacked a reasonable excuse for such action. He thought it best to try to eliminate his caesar by tricking him without warning.
[3.5.4] He therefore sent his most trusted imperial messengers to Britain with secret orders to hand Albinus the dispatches openly if they were admitted to his presence. They were then to ask him to meet them privately to receive secret instructions; when Albinus agreed to this and his bodyguards were not present, the messengers were to attack him without warning and cut him down.
[3.5.5] Severus provided them with deadly poisons so that, if the opportunity presented itself, they might persuade one of his cooks or cup bearers to administer a dose in secret.
[3.5.6] Albinus' advisers, however, were suspicious of the emperor's messengers, and warned him to be on his guard against this cunning schemer. Severus' actions against Niger's governors had seriously damaged his reputation; after forcing them through their children to betray Niger, as has been related above, and after making good use of their assistance, he put them to death with their children after he had got from them everything he wanted. His actions on this occasion clearly revealed Severus' despicable character.
[3.5.7] The efforts of Severus now led Albinus to increase the size of his bodyguard. None of the emperor's men was admitted into the caesar's presence until he had first been stripped and searched for concealed weapons.
[3.5.8] Now when the messengers from Severus arrived, they handed over the dispatches to Albinus openly and asked him to retire with them to receive secret orders. But Albinus, suspicious, had the men seized, and, putting them to torture privately, discovered the entire plot; after killing the messengers, he prepared to resist his revealed enemy.