is the Year of our Lord Twelve hundred and Fifteen, and rumours have reached Christendom that a new Khagan has united the tribes of the steppes. This event is the first clue that the Mongols are coming. Soon after, the first invasion wave hits as the Ilkhanate Horde arrives. The horde arrives with many thousands of blood crazed horse warriors (exact number subject to change). These regiments form the shock force of the Mongol invasion. Some time after 1224, the second wave hits, further to the North and again with a huge force of shock regiments, the Golden Horde comes into play.
These events, though very likely, are not guaranteed to happen; there is a death date after which the Mongols will not arrive. So, on average this is what will happen, but in some games only one horde will arrive and there is a small chance that neither horde will arrive.
To help the Mongols on their way to mayhem they get a special invasion casus bellum. None of this pussyfooting around for a single duchy, Mongol Khans will settle for nothing less than everything. So the Mongols find it far easier to conquer than anyone else. Having the Mongols as a neighbour is designed to be an interesting experience.
So the Mongols have arrived and they have their shock troops. These troops do not reinforce but also do not suffer attrition. The only way to rob the Mongols of their power is to fight them. You need to essentially feed troops into the meat grinder to weaken the core of the Mongol horde power. Even better, you get someone else to do the fighting for you. Either way, only once those regiments are sufficiently weakened will the Mongols start to function like an ordinary country.
So there we have it. The Mongols are designed to add an interesting late game experience, especially if you happen to be at the Eastern edge of the Map...