(Lessons 62'64)
Dhul'Qarnayn is a title, meaning the one 'with two horns'. What the actual name of Dhul'Qarnayn was, or when and where he lived, is not certain, although people have equated Dhul'Qarnayn to several historical figures without any convincing evidence.
Allah had granted immense power and substantial means to Dhul'Qarnayn, who remained grateful to Him, and acted according to His will. He used to go on long expeditions, travelling to distant places in both the east and the west. During one such journey, he came upon a primitive tribe residing between two mountains. The people complained to him about two savage tribes, Yajuj and Majuj, that repeatedly ransacked their land, and brought great destruction upon them. DhulQarnayn, at the people’s request, constructed a massive barrier that kept Yajuj and Majuj away, but warned the people that the barrier shall one day be brought down.
Allah informs us elsewhere in the Quran that Yajuj and Majuj shall spread great corruption in the land towards the end of times: 'Until when Yajuj and Majuj are let loose and they break forth from every elevated place, and the True Promise [of Judgement] draws near…' (21:96'97)