OK, I am doing background on history of my setting, and now am writing background on Fomorian invasions - which, in my world, replace barbarian invasions. Now, important parts are these (based on combination of Irish legends about Fomorians and history of Western Roman Empire and barbarian invasions): So what occured to me is that above is rather similar to Muslim conquest of Iberia and later Spanish Reconquista. But there are some questions: 1) Military organization. I was originally going for Byzantine thematic system, since we do not have foreign elites taking over domestic population. But is that realistic? Seeing how there would have been local warlords which either cooperated with Fomorians or else managed to resist them (e.g. mountains), would this lead to development of feudalism or would the Empire be able to coopt them from the start? Or maybe there would be some mixture of thematic system and proper feudalism, seeing how central government never really disappeared though it was reduced in size? So maybe central army + thematic provincial armies + magnates retinues? What about possible religious military orders (I imagine "demonic-looking beings from under water would rather help their appearance)? 2) Social and cultural impact. Would Fomorian invasions lead to development of kinda-sorta chivalric ideals or warrior ethos? I do not imagine these would be concerned with honour as such, considering the context, but there might be some code developing anyway. Or maybe - considering how dangerous they were - invasions would lead to emphasis on avoidance of direct military confrontation, and focus on diplomacy? Or maybe even both? 3) Political impact. What is the likelyhood of the Empire splitting into two or three different states and having to be forcibly reunited later? And when would that happen? 4) Anything I am missing here?