Procopius was an official in the Byzantine government under Justinian and Theodora, who reigned from 527 - 565 AD. He wrote a lot of history--that is, he wrote official histories for the court, which of course flattered Justinian. On the side, though, he was working on a more personal project--a 'secret history' in which he spoke about his real opinions. And Procopius did not think much of the Emperor and Empress.
This is a short book that is filled with invective. Procopius paints Justinian and Theodora as endlessly grasping and cruel tyrants. Theodora's youth sounds like the most scurrilous gossip, and I don't see how Procopius could possibly have known what was true and what was false, but much of the rest of the material is what he himself would have witnessed. He portrays Justinian as voracious for cash, but a spendthrift; as weak and indecisive, unconcerned about the welfare of his subjects, and as totally corrupt and incompetent to rule. Theodora is shown as extremely cruel and vindictive.
An interesting read, obviously, but it does become difficult to tell how much is pure venom and how much is accurate. I have no doubt that Justinian, like most human beings given absolute power, was pretty awful. On the other hand, I do doubt that his head disappeared in a demonic cloud late at night, as one courtier claimed.