By the turn of the first millennium C.E., the romance of Alexander in Arabic had a core centered on the Greek legendary material ... Interwoven later into this narrative in the "Tales of the Prophets" literature were episodes of an apparent Arab-Islamic elaboration: the construction of a great barrier to keep the people of Gog and Magog from harassing the people of the civilized world until Judgement Day, the voyage to the end of the Earth to witness the sun set in a pool of boiling mud, and Dhu al-Qarnayn's expedition into the Land of Darkness in search of the Fountain of Life accompanied by his companion Khidir ("the Green-One").