Habib Ali Zainal Abidin at a majlis ta'lim at KL Ba'alawi last Sunday said:
There are many versions of the real name of Dhul-Qarnain. Some thought he was Iskandar al-Yunani, the man who built Alexandria in Egypt who lived in the times after Prophet Ibrahim. He was well known as Alexander the Great. One of Iskandar al-Yunani's advisors was Aristotles whose worldview did not conform to aqidatul tauhid. Therefore, it is impossible that he was the Dhul-Qarnain that was mentioned in the Holy Qur'an.
Dhul-qarnain was actually Iskandar al-Maqduni who lived during the reign of Prophet Ibrahim a.s. Allah did not, however, mention his real name but referred to his qualities, his akhlak and his deeds for us to emulate.
Allah granted Dhul-qarnain amazing strengths - inna makanna lahu fil ardhi. He was 'makmun indallah' - one who is trusted by Allah. Allah entrusted him to rule the earth. Allah had given such extraordinary power to only 4 persons. Two of them were mukmin and two were kuffar. The mukmins were Sulaiman and Dhul-qarnain and the kuffar were Buktunsir and Namrud.
When Dhul-qarnain arrived at a miry spring where the sun had set, Allah gave him an inspiration (ilham). He was given two choices, whether to punish or forgive them. But know that the sun never sets, it just appears to be set at one place yet it appears to be rising at another place. So the earth is always in zikrullah and salawat because some people are praying Isha while others are praying zuhur.
When Dhul-qarnain arrived at the place situated between two mountains (barriers/as-saddain) whose inhabitants spoke using sign-language, he was asked to help protect them. Dhul-qarnain agreed to help the dwellers build 'rodma' - some sort of two-layer high-walls. The barrier would last a long time until the time comes for Yakjuj and Makjuj to appear.
The above is a partial transcript of Habib Ali's lecture on tafsir of verses 83-88 of Surah Kahf. Below is the tafsir of the same verses, quoted from Tafsir al-Jalalayn, translated into English by Feras Hamza.
وَيَسْأَلُونَكَ عَن ذِي ٱلْقَرْنَيْنِ قُلْ سَأَتْلُواْ عَلَيْكُم مِّنْهُ ذِكْراً
And they, the Jews, question you concerning Dhū’l-Qarnayn, whose name was Alexander; he was not a prophet. Say: ‘I shall recite, relate, to you a mention, an account, of him’, of his affair.إِنَّا مَكَّنَّا لَهُ فِي ٱلأَرْضِ وَآتَيْنَاهُ مِن كُلِّ شَيْءٍ سَبَباً
Indeed We empowered him throughout the land, by facilitating [for him] the journeying therein, and We gave him to everything, of which one might have a need, a way, a route to lead him to that which he sought.فَأَتْبَعَ سَبَباً
And he followed a way, he took a route towards the west,
حَتَّىٰ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِي عَيْنٍ حَمِئَةٍ وَوَجَدَ عِندَهَا قَوْماً قُلْنَا يٰذَا ٱلْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْناً
until, when he reached the setting of the sun, the place where it sets, he found it setting in a muddy spring (‘ayn hami’a: [a spring] containing ham’a, which is black clay): its setting in a spring is [described as seen] from the perspective of the eye, for otherwise, it is far larger [in size] than this world; and he found by it, that is, [by] the spring, a folk, of disbelievers. We said, ‘O Dhū’l-Qarnayn — by [means of] inspiration — either chastise, the folk, by slaying [them], or treat them kindly’, by [merely] taking them captive.قَالَ أَمَّا مَن ظَلَمَ فَسَوْفَ نُعَذِّبُهُ ثُمَّ يُرَدُّ إِلَىٰ رَبِّهِ فَيُعَذِّبُهُ عَذَاباً نُّكْراً
He said, ‘As for him who does wrong, by way of [practicing] idolatry, we shall chastise him, We shall slay him. Then he shall be returned to his Lord and He shall chastise him with an awful chastisement (read nukran or nukuran), that is, a severe [one], in the Fire.وَأَمَّا مَنْ آمَنَ وَعَمِلَ صَالِحاً فَلَهُ جَزَآءً ٱلْحُسْنَىٰ وَسَنَقُولُ لَهُ مِنْ أَمْرِنَا يُسْراً
But as for him who believes and acts righteously, he shall have the fairest reward, namely, Paradise (the annexation construction [jazā’u l-husnā, ‘the fairest reward’] is explicative; a variant reading has jazā’an al-husnā, ‘[he shall have] as a requital that which is fairest’; al-Farrā’ said that this accusative [reading of jazā’an] is [intended] as an explanation [of the nature of the requital] by way of attribution [to ‘that which is fairest’], and we shall speak to him mildly in our command’, that is to say, we shall command him with what he will find easy [to bear].