They said: "O Zul-qarnain! the Gog and Magog (People) do great mischief on earth: shall we then render thee tribute in order that thou mightest erect a barrier between us and them? Al-Kahf 18:94 ⧉
Tafsir:
Probably Dhu’l-Qarnayn’s third campaign was in the north east, in the direction of Iran. He reached a place where savages were living. Because they could not have any interaction with any other community, they were hardly able to understand any other language but their own. The ‘two mountains’ probably lay between the Caspian Sea and the Black Sea. Here savage tribes used to invade from other places and would then flee through mountain passes after indulging in plunder. Dhu’l-Qarnayn constructed an iron wall between the two mountains.