I triggered the event on the Roman empire, to see what would happen, and after the initial province going to the zombies... nothing did. For years the zombie province existed doing nothing while I got event after event about the zombie menace and unity and all that jazz. But not a single unit of zombies existed. So, I finally sent in a unit to seige the place and see if that would do anything, and all that happened was the game crashed as soon as the siege ended.