Sudanese Islam is organised through Sufi brotherhoods to a degree unusual anywhere, and the leading families of those brotherhoods are simultaneously political dynasties. The Khatmiyya, led by the Mirghani family, and the Ansar, descended from the followers of the Mahdi, have supplied the two poles of Sudanese politics for more than a century, with parties attached to each. A Sudanese election has often been, in part, a count of tariqa allegiance.
The Mahdiyya is the central event. Muhammad Ahmad declared himself the Mahdi in 1881, built a movement that took Khartoum in 1885 and killed Gordon, and established a state that lasted until Omdurman in 1898. His tomb in Omdurman remains a focal point, and the Ansar are still a mass organisation.
Before all of that, the middle Nile was Christian for some eight centuries. The Nubian kingdoms of Nobatia, Makuria and Alodia adopted Christianity in the sixth century and maintained an ecclesiastical culture in Greek, Coptic and Old Nubian, with cathedral churches whose wall paintings survive: the frescoes recovered from Faras before the Aswan flooding are now divided between Warsaw and Khartoum, and Old Dongola remains one of the most important Christian archaeological sites in Africa. Islamisation came gradually through migration and trade rather than through conquest.
Traditional practice continues in the Nuba mountains and among other peoples of the peripheries, often alongside Islam or Christianity rather than instead of them, and it was those peripheries that bore the worst of the wars.