A key to defeating the Fatimids when I've fought them was to have a strong navy to transport your (hopefully near 10k at the very least) army to one of their remote counties. You'll probably have some allies available or else you did it wrong. Try to bribe the Fatimids into attacking somewhere else by waiting until there is about to be a succession there and sending the heir a gift. If you are lucky like I was he will be a minor when you move into the area. I sent him some Eastern European wives as betrothal and 20 gold and he's still distracted elsewhere now that he is an adult. I also keep my chancellor there to tell him I'm a really cool guy. Also if you can invite a lot of holy men to your court, you can sort of stun lock him by offering to marry your courtiers to his female courtiers. He runs out of female courtiers and princesses eventually though. He doesn't seem picky as to who he marries princesses to either as long as they can make babies eventually. The best thing is you get to keep the babies. I stalled him for long enough that way to have my Chancellor talk him out of an inevitable invasion of Israel. Some of his Princesses were really well educated too, and are now teachers for the next generation of landless cousin Piasts who might benefit from walking like an (Catholic) Egyptian in the off chance I find an opportunity to take the County of Cairo and all it's sweet, sweet holdings without having all the Shia nations jumping on me for my treachery.
Polrusalem lives on, though I'll probably wait for a few patches while they work on factions. Hopefully I can make my own, though maybe their thought is to require the expansion for that which is fair enough. I admit I screwed up in changing both crown laws and inheritance laws in Hungary and Poland on the ruler when he was likely to die and has a weird succession; but I thought his badasserry would make up for it and I could make a group of loyalists of the new generation who weren't ruling before I changed both laws at once. The minority younger generation of vassals who weren't in power at the time of approval of both the crown law change and the change to Primogeniture adore my 21 diplomatic but otherwise mediocre character. The older generation have despised him ever since he passed the laws.
The King is Hungarian Piast due being the son of the first Piast king of Poland who was himself the son of the first playable King of Poland. This King was born and educated in Hungary to a Hungarian mother and Polish turned Hungarian father. He and his father both were controlled by the AI for significant amounts of time, including his childhood education. As such, his stats are pretty terrible except for 21 diplomacy. Piasts control nearly everything East of Germany that isn't Muslim, though that AI part of the dynasty keeps breeding itself out of the Kingship of Rus by ending up with normal marriege princessess inherit and thus the children are non-Piast; which is of course unacceptable to the AI controlled Piast Dukes which results in having Rus-Piast-Queen brothers, nephew or cousin Dukes retake the crown through wars the next generation. Some Piasts minorities control Volga Bulgaria somehow; I assume through swapping relatives with Rus. Much of Volga Bulgaria, including the capital, are still Tengri which means eventually someone will probably take their lands though they are extensive for a half pagan state. If it's Rus that's not a problem since they are Piast also, but if it's those pesky Byzantines or Muslims who go expanding moderate power-heathen-ways my dynasty in Volga Bulgaria could either be wiped out eventually or become the Byzantine Emperers. Most likely though they'll be taken by Rus or the Horde if they ever arrive to face Ultra-Poland.
The weird succession is that the King's first son died after the king became king of Hungary and Poland and some lands in Italy, and also some in Russia and Finland which I grabbed while everyone was still beating up heathens. After the primary son died though, I earned Jerusalem from a Pope's Crusade, and also claimed Sicily on my own. I have another Crusade granted kingdom in either France or Spain but I can't recall which as it's brand new. I joined the timely Crusade because the vassals in Hungary and Poland were forming huge factions to take my crowns or become independant. These factions are made up almost solely of those Nobles ruling previous to my raise crown and set primogeniture succession law changes in as many of my Kingdoms as I could.
The problem is that the newer Kingdoms are split between all the sons. The old Kingdoms go to various grandsons or great grandsons. The heir I will continue as is going to receive the Kingdom of Jerusalem and the two duchies the King has in Israel. He also has a portrait of a Middle Easterner because the second wife of the King was a Fatimid Princess. Unfortunately, either because I don't have Sword of Islam or because it doesn't work on Catholic Hungarians, the heir does not show any sign of genealogical holiness. The Fatimid Princess also lost her Shia traits when the king asked her to convert to Catholicism.
What I'm going to try to do is educate my heir until he is 16; then give him all my titles and hope the vassals are less pissed at him than they are at me until my by then 70ish year old ruler dies. In addition it allows me to keep all my Kingdoms and territories instead of most of them splitting off and going independant. After that the heir will hopefully be able to lay non religious claim to Egypt or a caliphate or something else significant the Fatimids own. If not it's holy war for Sinai or perhaps whatever is still Sunni in Spain. At some point maybe I'll convert to a Religion I'll hastily look up on wikipedia if I get the chance, if it sounds ok and I am powerfull enough to withstand a Crusade or Jihad.
I started this by starting as a Piast and betrothing my first heir to the daughter of the Hungarian King. I expanded Poland at the expense of the small non-Christians states whenever they were collaborating against some kind of invader, whether German, Danish, Norwegian, or Russian. The young Hungarian royal sons then met with a surprising amount of accidents due to their personalities, which made their spymasters susceptible to bribes. Eventually the heir became King of Hungary, Heir to Poland. Poland's first ruler had at every opportunity claimed land from the local Pagan states, the immediately north of which are on the sea. It's crucial for Poland to have a Navy large enough to transport it's army to the Mediterranean; because you probably married Matylda the Italian Duchess and your Heir will have half of Italy which is conveniently situated adjacent to Rome. I took Rome as a second generation Piast female ruler once; I should have tried to make a puppet Piast Pope but that was a different game, which I lost the save of. The very first thing to do as Poland is to plot to kill your inept brother. He just screws everything up, every single time. Plus, you are his heir. Then plot to kill as many childless Polish dukes as you can as they all have you as their error before they have children (one was homosexual the last time, I dunno if that's set, but you don't have to kill him as his heir is you.) I ended up with two minor Dukes managing to reproduce before I could slay them outrageously, and only one produced a male child before he died. That male child was next after I betrothed his sister to a son of the king, and then Poland was either owned by me or to be inherited by girls to be married to my male children. I even had a female child left over to send to matrilinearly marry a daughter of a Duke in Novgorad and her progeny ended up forming the Kingdom of Rus. I had to compete with his descendants for Tengri land near Siberia before it was all taken by Russian states. The exception was the fairly large Kingdom of Volga Bulgaria, which was also ruled by Piasts. Luckily, that particularly Tengri branch of the Piast tree did not seem to mind Piast Rus and Polrusalem conquering their fellow Tengri states.