Herodian is known only by his History of the Empire after Marcus, written in Greek, covering the period 180–238 in eight books. He seems to have been a minor Roman civil servant. He is widely pronounced to have been Syrian, but this is only an inference. By his own account, Herodian was a contemporary of the events he describes:

I have written a history of the events following the death of Marcus [Aurelius] which I saw and heard in my lifetime. I had a personal share in some of these events during my imperial and public service.
History of the Empire after Marcus I, 2

Though the eight books have survived intact, there are some indications that the text is a work in progress – that it is unrevised; perhaps incomplete.[*] For instance, Herodian first announces (I, 1) that the work will encompass sixty years (which it pretty much does), and then (II, 15) that he intends it to cover seventy years.[*]

The History is not an entirely reliable source. Herodian is rather too ready to modify the facts to enhance the story – a common comment is that he was a novelist rather than a historian.[*]

Translation by C.R. Whittaker

The various indications of an unrevised or incomplete text are itemized by H. Sidebottom: ‘Herodian’s Historical Methods and Understanding of History’, footnote 183, Aufstieg und Niedergang der Römischen Welt Vol. II.34.4 (1998).
A.R. Polley* considers that:
The easiest way to resolve this contradiction is to suppose that Herodian originally intended to write a history covering about seventy years, but was overtaken by old age, indisposition or indolence as he approached the sixty-year mark, and so finally decided to stop at the relatively convenient point of 238, remembering to alter his ‘preface’ but failing to notice the statement which occurred later in the work.
* ‘The Date of Herodian’s History’, L’Antiquité Classique Vol. 72 (2003), freely available online. This paper was produced in response to a paper by H. Sidebottom – ‘The Date of the Composition of Herodian’s History’, L'Antiquité Classique Vol. 66 (1997), freely available online – in which it was argued that “Herodian may well have been less than totally honest” when he claimed to be a contemporary of all the events he writes about – he “could have been born during the reign of Commodus or even later” – and that the History was composed during the period 260–268.  A.R. Polley was unimpressed by the argument, and concluded: “There is no good reason, despite Sidebottom, to dissent from the communis opinio, that Herodian wrote in the 240s or very early 250s.”
Harry Sidebottom is a sympathetic critic:
The reader’s engagement with the text is seen to be fostered by a variety of reasonably sophisticated narratological and rhetorical devices, which play with certain superficial levels of historical 'truth’ in order to convey with greater immediacy what the text sets up as more profound levels of historical 'truth’.
‘Herodian’s Historical Methods and Understanding of History’,
Aufstieg und Niedergang der Römischen Welt Vol. II.34.4 (1998)
Anthony R. Birley, though, is brutal:
He [Herodian] has always had his fanciers, or defenders, but he was careless, ignorant and deceitful, a self-conscious stylist who wanted to write a ‘rattling good yarn’ and happily adjusted the facts to achieve readability and excitement.
Septimius Severus: The African Emperor Revised Edition (1988),
Appendix 1