I have lately been using the term “The Brythonic Heroic Age”. This as far as I can tell is my own invention. I thought I would do a short post about why I am using it, and what it is.
Others use their own terms for this period, the main being the “Sub-Roman Era” or some variation thereof, however I think this is a poor representation of the era. Others like John Koch have lumped this into a greater “Celtic Heroic Age” as seen in his book “The Celtic Heroic Age: Literary Sources for Ancient Celtic Europe and Early Ireland and Wales”. It is understandable to lump this era into both the greater Celtic group as a whole, and the Roman era, however I think it is also appropriate that it has a more focused name when discussing it alone. The most fitting in my opinion is something that addresses the fact that many of the formative heroes of the descendants of the Britons lived (or are attributed to have lived) during this period. Thus The Brythonic Heroic Age. A more evocative name than “sub-Roman Britain” and more specific than “The Celtic Heroic Age”
So what is this era? This is the period roughly from the exit of Rome (or expulsion, it's complicated) around 410 A.D. to the establishment of the major Anglo-Saxon kingdoms starting with Æthelfrith’s establishment of a united kingdom of Bernicia and Deira around 604. It was during this era that Romano-British identity began and faded, when heroes like Caradog Vreichfras, Urien of Rheged, Cadwallon Lawhir, Rhydderch Hael, and Arthur himself would have lived. I think it fitting that these men fighting against internal and external threats deserve an era of their own, separate from the Romans they so revered, and the Irish they fought, and intermarried with.