This is a slightly expanded version of my response to the Fixer-Upper Challenge Episode of D-Infinity Live, in which I took Vampire: The Dark Ages, and made it something different. I will probably develop this enough to run it for a future D-Infinity Plays episode! Maybe even for publication, in the fullness of time. I mean: monsters truing to thrive and survive the dissolution of the Roman Empire and the total upheaval of their worlds and the worlds of men?! It works as a setting!
System: The Ragnarok Engine. My system, with the following modifications:
Now that I think of it, I actually wrote a story set in very close to this sort of setting. You can read "Their Blood is the Sea" in the 6th volume of D-Infinity Magazine! I will see about getting it posted to D-Infinity in the near future.