Tafsir of Al-Kahf 18:98

قال هذا رحمة من ربي

(Dhul-Qarnayn) said: "This is a mercy from my Lord...'' meaning, after it was built by Dhul-Qarnayn

قال هذا رحمة من ربي

He said: This is a mercy from my Lord, for the people, when he placed a barrier between them and Ya'juj and Ma'juj, to stop them from spreading evil and corruption on earth.

فٳذا جاء وعد ربي

but when the promise of my Lord comes, means, when the true promise comes.

جعله دكاء

He shall Dakka' it down to the ground. means, will make it flat. The Arabs use Dakka' to describe a female camel whose back is flat and has no hump. And Allah says:

فلما تجلى ربه للجبل جعله دكا

So when his Lord appeared to the mountain, He made it Dakkan. (7:143) meaning, level to the ground.

وكان وعد ربي حقا

And the promise of my Lord is ever true. means, it will come to pass without a doubt