In your first playthrough, you will immediately look to the east, you'll ask, 'I don't see a Mongol realm, where are they?' The Mongols don't start with provinces, they are known as the Illkhanate, they will attack one of the realms at the Southeastern edge of the map, most likely the Seljuks or Khiva.

A few months later, you will recieve a second message, the Scourge has arrived!! At around 1216 - 1240 (In my experiences), you will recieve a message detailing the rumours of a Mongolian Khan uniting all the tribes and are heading your way, if you studied your history you will know that these rumours are unfortunately true. If you start from the very beggining at Stamford Bridge, you will get almost 155 years to prepare for the atrocities that will soon arrive from the East. And as an end to this introduction, I will quote the words of Captain Kirk, Star Trek Wrath of Khan 'KHAN!!!!!!'. There are many obstacles for the Mongols, but in this page, you will understand that no obstacle is too large for the Horde. As a ruler or vassal of the West, it is your duty to protect the realm from this seemingly neverending horde of Steppe Wolves. In almost every game of Crusader Kings II, there will come a time when an army of vicious pillagers known as the Mongols will come to seek conquest and riches in the West.

Crusader kings 2 how to play as a horde