It sounds intriguing as a concept, but a few things confuse me. The dark ages is the period of time after the decline of the Western Empire, and the barbarian tribes are starting to form into proper kingdoms. Think feudal lords, Pope Gregory, etc. etc. Then you say you're borrowing from King Arthur, which takes place in an entirely different time period. Then you say you might go as far back as 400 C.E., over 70 years before Rome as a country actually fell. Then most of your plot synopsis entails that this RP only takes place a few years before Rome falls. 

The concept is certainly an enticing one, but could you clear up the discrepancies in the time period and setting?