I'm sure many people have found this already, but I thought I'd post the recipe for a bloodbath:
1)Find the Flambeau in the Forum and the Necromancy in the Catacombs. The Zampogna in the forum is helpful too, but not necessary.
2) Go to the area you want to conquer.
3) Kill one or two Romans, then light their corpses on fire.
4) Resurrect the flaming corpses. This means that everyone the zombies touch will catch on fire. The corpses should then kill more Romans, whom you can resurrect to kill more Romans. Rinse and repeat until you have an army of flaming skeletons.
4) Stand around and watch the carnage until the game tells you you have a bloodbath.