Death

It began as an accident. The Mongol invaders had planned to bring death to the Arabs, but they did not plan to cast it among their own armies. Death came anyway, and took nearly all of them. However, Death did not win that battle.

Instead the army invited Death to join their armory, and they catapulted him from one camp to the other. First, fortresses fell in Deaths wake, their guards succumbing to black sores, pale skin, and fever. Then ships came and carried Death as a stowaway, as some say. Others says the crew tried to take Death prisoner. Either way, Death sailed across the Black Sea and conquered Europe.

First Turkey fell, then Italy, then the Alps, then France, then Germany, and then Brittan. Death planned to push up into Poland and Russia, but he lost steam. Or, as some theorists say, he lost the motivation to continue. Other sources say he retired and abdicated his empire to a worthy opponent.