I am reading Mary Stewart “The Wicked Day” again and one thing I found of interest is that she regretted not having done more research before writing her earlier books because if she had she would have had Mordred and Arthur dying in battle against the Saxon invaders as she thought that was more historically likely.
So what did happen to Riothamus? Did he die in battle against Saxons, perhaps even in France, or die of old age, or an illness?