Allied with the North through bloodlines and alliances; they are a large part of the Mongol's attrition. Also pretty much neutral with HRE and the Pope. I'm going to have to fight defensively by continueing to declare war while they are busy absolutely destroying everything in the South East.
Fun Stuff I tried: I gave the Pope a few provinces out East right in the way of the Golden Horde. This gave me a way to support that inevitable war and he gave me some pope golds later for it I think. I've been building universities and cities and schools in Poland, so we out tech the Mongols but not the Muslim lands they are taking over. They are reforming that really strong Muslim nation that is usually the pain in the ass in the region, (you know the one) but as Tengri or whatever the Ilkhanate is. In addition they have all those Eastern counties too. They've come to the point they can recruit province troops instead of relying on doomstacks. This worries me greatly since they are currently the largest nation, beating even a Holy Lands engorged HRE.
EDIT2: Also most of north Mongol Land is Catholic or Orthodox. Whatever religion the Mongols convert to eventually (I'm trying for Catholic) will probably nail the others. Catholics are Strong everywhere in Europe, Orthodox is weak because the current Mongols are Tengri but much of the Mongol lands are Orthodox, so perhaps they will become Orthodox Mongols. I've been very active diplomatically with them even since before killing the Ilkhan and actually have a branch of Piasts crossed with the Ilkhanate and vice versa. That's probably why they've always invaded the Muslim lands, which are taken all the way to Jerusalem, HRE is the holder of that and they have been slowly griding away at the ilkhanate. I'm waiting for the Pope to be smart and declare a Crusade against the Mongols. I have left Catholic lands in Mongol control, so there is a chance, but he's picking on puny Muslim counties and islands in Spain. The Continuous Spanish Crusades only earned Europe in this game an enormous Ilkhanate to the East and a rapidly shrinking Holy Land to the Ilkhanate.
The Holy Roman Empire is the owner of Jerusalem, and I can join their wars. The Ilkhanate can recruit from it's early conquered land for first time and still has at least one doom stack still alive.
EDIT3: I also gave a bunch of counties to the Teutonic Knights for more Mongol attrition that would make new countries on my border I could kill a small doom stack over.
Now though I'm about to have one of those succession facepalm moments; where I'm going to have to give away a ton of land to not have everything disintegrate. That means I have to ought to invade Heathens, who are the Ilkhanate. Somehow that pope needs to do a Crusade pointed in the right direction; so I can both have a lot of help with a long war to grind down the death stacks. I have a ton of diplomacy skills in my family, so it would be nice if I could get one of them to send a nice letter to the Pope to make him declare a Crusade on the Mongols.