The word Khorasan strikes a chord deep in the heart of jihadists. One of the sayings (hadiths) attributed to the Prophet Muhammad says: “There will emerge from Khorasan black banners which nothing will repel until they are set up in Jerusalem”.
This is part of a prophecy which leads to the Mahdi (Messiah) showing up and the subsequent End of Days. The ISIS flag is black based on the belief that Muhammed himself flew such a banner. So, if you are in ISIS (Khorasan) you might believe that you are in the vanguard to establish a caliphate in the region, leading to global domination, and the ultimate goal – ushering in the end of the world.