Ok, the title turns out to pretty much make any summary I can do irrelevant as it grabs all the salient points. Nonetheless…
Marcus‘ son Commodus succeeded him and three years later, in 183, his sister Lucilla attempted to have him killed but was herself exiled and then executed. The attempt was falsely made in the name of the Senate but it still made him suspicious of them. As a result, many senators and their families were murdered. Eventually a plot by his favorite concubine and head of the Praetorian Guards had him poisoned and then strangled in his sleep.
A senator born of slaves, Pertinax, was chosen to succeed Commodus. He recalled from exile, and pardoned, many of those who had experienced the wrath of Commodus’ suspicion. Many administrators of the empire, as well head of the Praetorian Guards, Laetus, were angered by the fact that Pertinax wasn’t quite so loose and luxurious with them than Commodus had been and attempted to have him killed. He failed but a mob of guards later seiged the palace and brutally killed him. Pertinax’s reign lasted 86 days.
The only viable way to preserve a constitution against the tyranny of an aspiring prince is to have governence by constitutional assemblies of the nobility and armed “commons.” Augustus convinced the Senate to give him (and his successors) lifetime power to head the civil, military, and tribunal offices in addition to being head of the religious authority as supreme Pontiff. This essentially made him the law in addition to being the law. Even with so much military and civil authority being vested in one person, for 200 years from August to Commodus there was virtually no outbreaks if internal bloodshed htrough military might.
In order that the Empire wasn’t left headless upon the death of an emperor, he was able to “adopt” another man as his son in the line of Caesar and vest with them some, or all, of the powers of the empire. During Hadrian’s time, he chose a fifty year old senator named Antoninus Pius and a seventeen year old boy named Marcus (later Marcus Aurelius) to succeed him. This pair was known as the “Antonines” and ruled for 42 years with wisdom and virtue. The reign of Nerva through the reign of the Antonines could be considered the happiest the world has known