The Iohannidos, or Song of John, of Flavius Cresconius Corippus, is unusual is several ways: an epic poem in Latin in the Vergilian style, celebrating the warlike deeds of a hero, it was written in the mid to late 6th century, almost 2 centuries after the last known ancient examples of Classical Latin. Based on its style, it must be considered Classical Latin; however, based on its date, it was composed well into the Medieval era. It was written in or around Carthage. Much else written in the same time and place has survived to our day; however, almost all of it is Christian theology, as different from the Iohannidos as can be. Most earlier Latin epics such as Vergil's Aeneid included copious references to the pagan gods. These are lacking in Corippus' poem. Some have pointed to this as evidence that Corippus was Christian. I'm not convinced. A pagan or an atheist in the 6th-century Roman Empire would have been taking a big risk by letting his true beliefs show.
The hero of the poem is John Troglita, a general who served under the Byzantine Emperor Justinian I during Justinian's short-lived reconquest of much of the former Western Roman Empire. John led a series of successful battles against nomadic peoples in northwestern Africa, including the Berbers. Corippus' poem follows these battles up until AD 548. Historians value it for its accuracy of detail, confirmed by comparison with writers contemporary to Corippus and by what we know of the modern-day Berbers.
It is unusual in being an account of Justinian's war written in Latin, as opposed to Greek.
Also, the way in which the text of the Iohannidos has been transmitted to us is unusual. It was known that Corippus had written such a poem; however, it was lost until the year 1814, when Cardinal Mazzucchelli, librarian of the Biblioteca Ambrosiana in Milan, came across a 14th-century manuscript of it. Before this, another of Corripus' works was known: In laudem Iustini minoris (In Praise of the Younger Justin), which portrayed the death of Justinian I and the early part of the reign of his successor, Justin II. This work was published in the late 16th century from a 9th- or 10th-century manuscript. No one seems actually to have liked as a piece of writing by. It is, frankly, a rather disgusting piece of servile flattery. The hope to gain the favor of the powerful is clearly its entire object on the part of the author; nevertheless, it is very well -written, and, like the Johinnodos, it contain much information of interest to historians -- in this case, information about the 6th-century court at Constantinople and its manners ans customs. (And I suppose it's possible that the powerful men Corippus flattered in the poem actually enjoyed reading it.)
Having been the only known work by Corippus for a long while, the In laudem Iustini minoris in no way prepared readers for the great positive surprise of the Iohannidos, which is a great pleasure to read, with its polished style conveying a rousing tale of action and adventure with convincingly lifelike characters. People would want to read this poem even if it contained no historical interest at all. The great help it lends to historians is gravy. The great interest to scholars of Classical Latin, finding such a polished work so written so late after such things had been assumed to have died out, is gravy. Is it as good as Vergil? Well, no. But if you like Lucan's epic poem of the struggle between Julius Caesar and Pompey, you might very well like the Iohannidos just as much.
Oh, and one other unusual thing about the Iohannidos: the only edition of it published since the end of the 19th century of which I am aware was put out the Cambridge University Press in 1970, edited by J Diggle and FRD Goodyear. It is the only book of which I know, amongst those published by Cambridge, which is so recent and which is all in Latin, from the title page to the dedication to the Praefatio to the Index Historicus et Geographicus. So, yay!