The Historia Augusta is without question or rival the most enigmatic work that Antiquity has transmitted.
Sir Ronald Syme The Historia Augusta: A Call of Clarity (1971) Chapter 1

The HISTORIA AUGUSTA is a collection of Latin biographies of 2nd and 3rd century Roman emperors, heirs and usurpers. It was given its title by French classical scholar Isaac Casaubon, who published an edition of the work in 1603. The biographies begin with Hadrian (117–138), though his may well have originally been preceded by Nerva and Trajan (picking up from where Suetonius left off), and end with Carus, Numerian & Carinus (282–285). There is a lacuna where Philip the Arab, Decius, Trebonianus Gallus, Aemilian and the greater part of Valerian’s biography ought to be (the period 244–260).

SubjectAuthor
HadrianAelius Spartianus
Lucius AeliusAelius Spartianus
Antoninus PiusJulius Capitolinus
Marcus AureliusJulius Capitolinus
Lucius VerusJulius Capitolinus
Avidius CassiusVulcatius Gallicanus
CommodusAelius Lampridius
PertinaxJulius Capitolinus
Didius JulianusJulius Capitolinus
Septimius SeverusAelius Spartianus
Pescennius NigerAelius Spartianus
Clodius AlbinusJulius Capitolinus
CaracallaAelius Spartianus
GetaAelius Spartianus
MacrinusJulius Capitolinus
DiadumenianusAelius Lampridius
ElagabalusAelius Lampridius
Alexander SeverusAelius Lampridius
Maximinus ThraxJulius Capitolinus
Gordian I, II & IIIJulius Capitolinus
Pupienus & BalbinusJulius Capitolinus
Lacuna
ValerianTrebellius Pollio
GallienusTrebellius Pollio
Thirty pretendersTrebellius Pollio
Claudius GothicusTrebellius Pollio
AurelianFlavius Vopiscus
TacitusFlavius Vopiscus
ProbusFlavius Vopiscus
Four pretendersFlavius Vopiscus
Carus, Carinus & Numerian  Flavius Vopiscus

The biographies purport to have been written by six, otherwise unknown, authors, during the times of Emperors Diocletian (284–305) and Constantine (306–337), to whom the writers sometimes address remarks.* The generally held belief today, however, is that the Historia Augusta is the work of just one author, writing about a century later than the purported date.

Replete with dubious personages and bogus documents, the Historia Augusta appears to be an enormous prank – a whimsical blend of fact and fiction that has to be treated with caution.* Indeed, the last of the supposed authors, Flavius Vopiscus of Syracuse, mischievously reports a discussion about his predecessor, Trebellius Pollio, between himself and one Junius Tiberianus:

… Tiberianus asserted that much of Pollio’s work was too careless and much was too brief; but when I said in reply that there was no writer, at least in the realm of history, who had not made some false statement, and even pointed out the places in which Livy and Sallust, Cornelius Tacitus, and, finally, Trogus could be refuted by manifest proofs, he came over wholly to my opinion, and, throwing up his hands, he jestingly said besides: “Well then, write as you will. You will be safe in saying whatever you wish, since you will have as comrades in falsehood those authors whom we admire for the style of their histories.”
Historia Augusta ‘The Deified Aurelian’ 2

Nevertheless, the early major biographies – those of Emperors Hadrian, Antoninus Pius, Marcus Aurelius, Lucius Verus, Commodus, Pertinax, Didius Julianus, Septimius Severus and Caracalla – are considered to be reasonably well based in fact*. The biography of Elagabalus (called Heliogabalus by Aelius Lampridius) sets off in similar vein, but about halfway through turns into a work of fiction. The rest are predominantly figments of the anonymous writer’s imagination.

Historia Augusta translation by David Magie.

For instance, Julius Capitolinus concludes his biography of Macrinus: “… these [facts] we have gathered together from many sources and have presented to Your Serenity, Diocletian Augustus, because we have seen that you are desirous of learning about the emperors of former times.”  Whilst Aelius Lampridius, in his biography of Elagabalus (§34), comments: “It may perhaps seem strange to some revered Constantine, that such a scourge as I have described should ever have sat on the throne of the emperors …”
The HA [Historia Augusta] is a genuine hoax.… The text discloses a rogue scholar, delighting in deceit and making a mock of historians. Perhaps a professor on the loose, a librarian seeking recreation, a civil servant repelled by pedestrian routine.
Sir Ronald Syme ‘Controversy Abating and Credulity Curbed’
London Review of Books Vol. 2, No. 17 (4th September 1980)
… no good contemporary narrative survives for the critical middle fifty years of the third century, so that we must depend on the often fanciful and trivializing Historia Augusta, which reads rather like a gossip column in a tabloid newspaper, and once read is hard to forget.
Averil Cameron The Later Roman Empire (1993), Chapter 1
The change in opinion over the past century, from viewing the HA as a quagmire to considering it a treasure trove of intriguing information about later Roman cultural sensibilities, is welcome. But as a historical source, this work must always be approached with caution. Indeed, one cannot escape the feeling that even if the author(s) did not intend it as a joke, the HA has nonetheless amounted to one played on posterity.
Gavin A. Sundwall ‘Scriptores Historiae Augustae’
Encyclopedia of Historians & Historical Writing Vol. 2 (1999)
… and contain a great deal of authentic information, much of it not provided by any other literary sources but confirmed by epigraphic evidence.
Anthony Birley Lives of the Later Caesars (1976), Introduction