حَتَّىٰ إِذَا بَلَغَ مَغْرِبَ الشَّمْسِ وَجَدَهَا تَغْرُبُ فِي عَيْنٍ حَمِئَةٍ وَوَجَدَ عِندَهَا قَوْمًا ۗ قُلْنَا يَا ذَا الْقَرْنَيْنِ إِمَّا أَن تُعَذِّبَ وَإِمَّا أَن تَتَّخِذَ فِيهِمْ حُسْنًا (86)
(18:86) till he reached the limit where the sun set, *63 and found it setting in black waters, *64 and there he saw a people. We said to him, "O Zul-Qarnain! you have the power to punish them and also the option to treat them generously". *65
*63) "The limit where the sun set" does not mean the "place" of the setting of the sun. According to Ibn Kathir, it means that he marched to the West conquering one country after the other till he reached the last boundary of the land, beyond which there was ocean.
*64) "He found the sun setting in black muddy waters of the sea": if ZulQarnain was Cyrus, then that place would be the western limit of Asia Minor and the "black waters" would be the Aegean Sea. This interpretation is supported by the use of the word "`ain"instead of "bahr" in the Qur'an.
<*65) "We said to him" does not necessarily mean that Allah directly revealed to him these words, and that Z, ul-Qarnain was a Prophet or was the one who received inspiration from Allah, and the same is the reasonable conjecture. This concerns the time when Zul-Qarnain had taken possession of the land as a conqueror and the conquered people were utterly at your mercy. Then Allah posed a question before his conscience, as if to say, "Now is the time of your trial. These people are utterly at your mercy, and you have the option either to behave unjustly towards them or to treat them generously."