So, recently, I've been reading on the Crusades from Islamic perspectives and the collapse of Seljuqid Empire. There's this guy, Malikshah, who was the sultan of Seljuk Empire at their peak with the aid of Nizam al-Mulk, after Alp Arslan conquest. He died in 1092, and his son(s) fought each other, first Mahmud then Barkiyaruq.
At 1099 Syria is divided into several rivalling principalities nominally loyal to Seljuk and the eastern Islamic world is busy killing each other. Fatimids just recovered from civil war and the Nizar Ismailis (those assasin folks) have broken up with Egypt because their guy for throne was lost. So the First Crusade can just waltz (relatively) easy to Jerussalem
But Malikshah's death is not natural. He was poisoned. What if he survive the poisoning?