this scenario may be considered as a continuation of "v century" scenario.
during the last years of roman empire, when rome was just a shadow of its ancient power, a new challenge came from the north, aparently to give the final hit to the dying empire...
atila, king of the huns, created a great empire of terror from the urals to the rhin, and challenged rome itself. all countries and peoples were systematicly defeated and slaved by his hun warriors...
when everything seemed lost, roman general flavius aetius forged an alliance between goths and romans, and defeated atila (which had invaded gaul with a 300.000 men army...) in the great battle of chalons (champagne, france) in 451 ad.
despite this, atila invaded italia next year, and only the death of atila in 453 ad saved rome and all europe from further devastations.
the history of rome from here is just a succession of weak puppet-emperors, behind them was the real power of barbarian chiefs. finally, in 476 ad the young emperor romulus augustus was dismissed by the chief of his guard, odoacro, who proclamed king of italia himself.
the western roman empire had fallen, but the eastern part continued until 1.453 ad, when sultan mehmet ii finally conquered the city of constantinople.