Perverted Inventions are used on her

Unknown to the Phantom Thieves, Akechi was still alive and going through the Metaverse. It didn't take him long to find what he was looking a full, a palace belonging to an gullible idiot. After that, he just commissioned some artists to draw a few realistic looking devices, found the palace ruler in the real world and showed it to him. It barely took any convincing to trick him into thinking the devices were real.

After that, he used a 3D painting to make a replica of the fake device and showed it to the idiot. Then he excused himself to the bathroom and snuck into the Metaverse and into the Palace. There, the device was real. It worked exactly like he wanted and he was able to bring the futuristic device to the real world where it kept on working.

He left the strangers place, letting him discover that the device he left back there was fake. It wouldn't matter now that he had a real version made from the cognitive world with him. If he needs to make another, he can just find another gullible palace ruler out there. Shouldn't be too tough.

He wasn't quite sure what to do with the device yet. He could sell it and use the money to enact his revenge against the phantom thieves... Or use the device for that. They'd never see it coming. After all, the device was something that technically didn't exist. It only existed now because someone temporarily believed it did and he was able to bring it into the real world and into existence.