(Al-Kahf) 18 : 91
كَذَٰلِكَ وَقَدْ أَحَطْنَا بِمَا لَدَيْهِ خُبْرًا
Thus; and We had encompassed all that he had in Knowledge.
Being an emperor and a believer, Dul-Qarnayn dealt with the Best with good ones and retaliated appropriately with the wrongdoers testifying the verse 16: 126.