Tafsir of Al-Kahf 18:87

(He said) — meaning Dhul-Qarnayn, to that prophet or to those of his elite entourage, after receiving the command of his Lord and choosing the latter of the two options presented to him, as he was guided to do:

(As for him who wrongs) his own soul and does not accept my call, persisting in the great injustice which is polytheism, (we shall punish him) by killing him. It is apparent that this was done by the sword. Ibn Abi Hatim recorded from al-Suddi that he said: His punishment was to place them in copper bulls and kindle fire beneath them until they were severed within them; however, this is far from authentic. He used the pronoun of majesty (the royal 'We') according to the custom of kings, and the attribution of the punishment to himself is because he is the cause and the one who ordered it. The claim that it issued from him essentially is extremely far-fetched.

It is said that he intended by the pronoun both Allah and himself, and the attribution is based on the concepts of creation and acquisition; this is also far-fetched, in addition to the implications of associating Allah with another in the pronoun, and the controversy regarding that which you already know.

(Then he shall be returned to his Lord) in the Hereafter, (and He will punish him) therein (a terrible punishment) (87), meaning a detestable, horrific punishment, which is the torment of the fire of Hell. The word (punishment) is in the accusative case as an infinitive noun (masdar) for "He will punish him." It is also said that both "He will punish him" and "we shall punish him" contend for it. The intent behind "terrible punishment," with regard to the former, is what was narrated from al-Suddi, though this is contrary to the apparent meaning, as is not hidden.

In His saying (to his Lord) rather than "to Me," there is an indication that the preceding discourse was not by way of revelation to him, and that his debate was with the prophet or with his elite entourage.