an armed force of muslim denomination sacks Rome around 800 AD
it can come from iberia after a more sucesfull campaing in southern france or it can be an expansion of muslim rule in the apenines, or part of a large move into europe
or it can simply be a fluke, a army of saracen pirates caching the Romans off guard, during some non related political/religious turmoil or somesuch crisis
but preferably a large well organised military campaing, the above mentioned sounds abit like ASB
bonus if most of the apenines including Rome stay muslim, or at least if it is not taken back by christians for a few decades