A plan can be made perfectly, but then many things can go wrong after. One of the only ways to make a plan successful is to be the one holding the most information and account for all the possibilities.
The possibility that a lone Demon King that was proficient in spying and quick movements would come to collect information about the place where a momentous event occurred was high.
The question after was how would you know who this Demon King was, and even possibly get your hands on them. The memories from a close-knit individual from their ranks gave all the answers, and brought about the outcome occurring now.
