Supreme Pontiff Israfel, Catholic Church
Surrounded by the City of Rome located in Italy, the Vatican which was the headquarters of the Catholic Church was the most powerful faction in Europe.
They began as a relatively small group of no more than a few thousand, but their power was so overwhelming that even the terrifying zombies and mutant beasts were easily suppressed during the early days of the apocalypse.
The power that the Catholic Church revealed was one that humans should not have, and even the modern armies eventually submitted to their magical arts.
Using abilities passed down from ancient ages and learned over many years of effort by people they considered chosen by the heavens, the priests and holy warriors of the church not only withstood the zombie assault but in less than a month managed to recapture a good portion of Rome and save a great number of lives.