The statement in verse 86: حَتَّىٰ إِذَا بَلَغَ مَغْرِ‌بَ الشَّمْسِ (until when he reached where the sun sets) means that he reached the far limit towards the West beyond which there was no populated area.

The word: حَمِئَةٍ (Hami'ah) in the succeeding phrase: فِي عَيْنٍ حَمِئَةٍ (into a miry spring) literally means dark marsh or mud carrying the sense of water beneath which there is dark mud and which causes the water it-self to appear black. As for the sense of his seeing the Sun setting into such a spring, it means that an onlooker perceived it as setting into the spring because there was no habitation or dry land in sight. This is like being in an open field while the Sun is setting where as far as one can see there appears to be no mountain, tree, or structure, naturally one who looks at the sight would feel that the Sun was sinking into the land mass.

Said in the sentence which follows immediately was: وَوَجَدَ عِندَهَا قَوْمًا (and found near it a people), that is, near this dark spring, Dhul-Qarnain found a people. The later part of the verse shows that these people were infidels. Therefore, as said in the next verses, Allah Ta’ ala gave Dhul-Qarnain the choice of punishing them right away for their infidelity. Or, if he so wished, he could choose to deal with them benevolently by first asking them to consider the message of faith and bring them round to embrace it through dissemination of information and good counsel. After that, he could reward those who believe and punish those. who do not. In response, Dhul-Qarnain elected to go by the second option. He said that he would try to bring them to the straight path through good counsel and advice. Then, he would punish those who stood by their infidelity and reward those who believed and did what was good.

The statement: قُلْنَا يَا ذَا الْقَرْ‌نَيْنِ (We said, 'O Dhul-Qarnain...) shows that Allah Ta` ala has Himself said this addressing Dhul-Qarnain. Now, if Dhul-Qarnain is taken to be a prophet, there is no problem here. It will mean that it was said to him through the medium of revelation. And if, his prophethood is not to be recognized, there is only one way to rationalize the statement: قُلْنَا (qulna: We said) and the address: يَا ذَا الْقَرْ‌نَيْنِ

Dhul-Qarnain). This way could be to take this address to have been made to Dhul-Qarnain through the medium of some prophet - as suggested by the reported presence of Al-Khadir (علیہ السلام) with him. Then, it is also possible that this revelation is just not the kind of wahy that is peculiar to a prophet or messenger of Allah. May be, it is a wahy or revelation in the literal sense like the word: وَاَوحَینَا (awhaina: We revealed or put into the heart) used in the Qur’ an for the mother of Sayyidna Musa (علیہ السلام) - though, there is no probability of her being a prophet or messenger of Allah. But, Abu Hayyan says in al-Bahr al-Muhit that the command given here to Dhul-Qarnain is a command to punish and kill those people. No such command can be given without the authority of a revelation to a prophet. This action cannot be taken on the authority of Kashf (illumination) and Ilham (inspiration), nor can it be activated through any other source without the authority of wahy (revelation) to a nabiyy (prophet). For this reason, no probability other than the one being mentioned here is sound: Either Dhul-Qarnain himself is taken to be a prophet, or that there may be a prophet present during his time and it is through him that Dhul-Qarnain is addressed. And Allah knows best.