In 825, Merfyn Frych (Marvin the Freckled) assumed the Gwynedd throne and started a new dynasty there. By 829 he had patronized the Historia Brittonum. Merfyn’s writers made use of a northern chronicle probably written down by Rhun the son of Urien, but as far as the first few decades after Rome left his historians were left with Gildas, and Bede.
Bede had used the Kentish Source to flesh out the character of Vortigern. Kent had used him to justify her primacy, while he had broadened that to the legitimacy of the Germanic peoples. Merfyn had different goals though. He probably didn’t know that Gildas had been guessing with the superbus tyrannus, but he did know that Vortigern, the name Bede had used, was the name of an important king in the Powys dynasty. That was important for him because his alliance with Powys was so close that he had married a Powysian princess. It wasn’t in his best interests to harm Vortigern’s reputation. In fact, he would have wanted to protect it. We can’t know what Merfyn put in his Historia Brittonum, no version of the original has survived. It was probably pretty positive though, with a good spin on the escapades of Hengest and Vortigern – Vortigern’s son might have been introduced just to keep the negative away from him. Who knows, maybe Arthur’s twelve battles were originally assigned to Vortigern.
A couple hundred years later, his descendent Hywel Dda was the most powerful man in Wales. He ruled from Dyfed instead of Gwynedd. He also had designs on Powys, not an alliance with it. When he got ahold of the Historia Brittonum he probably cut most of what Merfyn had and replaced it with Gildas and Bede’s stuff. Then he added a few more things to make it clear that Powys’ progenitor had been morally weak and religiously worse than a pagan.
Alfred also had access to the Kentish Source when he wrote The Anglo-Saxon Chronicle, but he accepted everything he found without editing it for his own designs. He also accepted any information he could find on Wessex and Sussex’s history, which is why we have such early dates for some their rulers. Some of the information is even very accurate, like the battles of the late sixth century. Of course Alfred didn’t really care what Sussex and Kent’s histories were or how true they happened to be. In his time, the Danes had conquered most of England and he was the only Anglo-Saxon king left. His Anglo-Saxon Chronicle was written as a sort of communal history for the Germanic people. All he cared about was showing them that they were united by a common history and culture so that he could united them under his banner and beat back the Danes. That was why he faithfully presented the people of Kent as the first leaders.
The fact is that Gwrtheyrn (his Welsh name) was a ruler of mid-sixth century Dubonnia – that’s around the Wye river in Wales. He was a powerful man in his own time, maybe even powerful enough to control England south of the Thames all the way to Kent. When Powys developed a dynasty it absorbed his names into their king’s lineage to make it sound more prestigious. It’s funny how one of Gildas’ broad titles blossomed into the pathetic figure we know today as the man who gave away England to the Anglo-Saxons.