The dense fog dispersed.
The already scarce sunlight of Gotham seemed even colder as it shone through the floor-to-ceiling windows on the top floor of Wayne Tower, but Robin King did not care. He could tell that the dense fog had truly lifted.
His first encounter with the descending fog, when he had nearly fallen from a skyscraper, had taught Robin King a valuable lesson. After the fog cleared, he used the devices in his belt to measure the magnetic field and various other readings here, determining what the world's data looked like outside the fog.
After entering the fog, he had also monitored the data within it. In the mysterious Mist Realm, all kinds of readings, including the magnetic field, differed from those of the real world.
So when Robin King took another measurement, he discovered that he had indeed returned to the real world.
Of course, the fog might have been capable of falsifying data, so he had another trump card: returning through the portal to see Laughing Man.
Having worked under Laughing Man for so long, Robin King knew quite well where this fusion of Batman and Joker drew his strength from. In short, no one could possibly outmaneuver him.
No illusion could imitate Laughing Man. Wherever he was had to be real. As long as Laughing Man was real, then the world he occupied had to be real as well.
Robin King came and went in a hurry, but it did not delay him for long. He also replenished the tools in his belt and brought back materials for constructing the brainwave intrusion device.
As he had said before, knowledge was not fake. Robin King knew that the principles behind the machine he had seen in the illusion were correct and entirely usable.
As for whether