The Barrier of Gog and Magog

The barrier is mentioned in Surat al-Kahf: Dhu'l-Qarnayn builds a wall of iron and نحاس between two mountains to hold back Gog and Magog, and it is opened in the last days when they pour out. The scholars say they are a people of the earth whom God restrained behind the barrier, and their emergence in the last days is among the signs before the Hour; reports place their coming after the descent of Jesus, when people flee to fortresses and places of worship.