When will the Great Slaughter occur, and who will be responsible for it? Will it be preceded by a confrontation with an enemy? And will the caliphate after it be a caliphate of justice or injustice?

The Great Malhama (the great war) is a tremendous war that will take place between Muslims and the Romans at the end of time, before the emergence of the Dajjal (Antichrist). These events will occur in succession: the Great Malhama, then the conquest of Constantinople, then the emergence of the Dajjal, then the descent of Jesus (peace be upon him) to kill the Dajjal, and then the emergence of Gog and Magog. The cause of this Malhama is the Muslims and Romans fighting a common enemy, followed by the treachery of the Romans. The leader of the Muslims in this Malhama is the Mahdi, as indicated by the categorization and narration of Mahdi-related hadiths by some scholars. The Caliphate will return to the prophetic methodology, and scholars have differed as to whether it will return before or concurrently with the appearance of the Mahdi.