So, I put the settings so that the Mongols could spawn in at any moment after 50 years, epidemics were more common and more deadly, and the Black Death was especially powerful. The year was 848, about 70-ish years after I started, and the Black Death AND Mongols spawned AT EXACTLY THE SAME TIME!

Needless to say, Europe has been completely fucked over. A combined assault from disease and horde, so early in the game, left everything destroyed. The Black Death mobilised first, spreading from Iberia to India in a year and staying relevant for about a decade. It has brought major families, such as the Karlings, down to a handful of members. Even worst, every province has been severely depopulated, which means less manpower. Which means less resistance for the horde that attacked straight afterwards. I'm playing as Ruthenia, and decided it was best to swear fealty than lose all my holdings to their Invasion. The rest of the world are in no shape to hold them off. Europe as I know it is destroyed.