Doppelganger magic was a form of demon summoning contained in the Demon King summoning document. Of course, rather than a demon, the doppelganger I used could only be seen as a minor alter ego with AI that copied my thought process. Even so, the structure of the technique was based on demon summoning, so it could not be called completely different magic. It was the magic of the so-called ship of Theseus. [1] 

"Why did you go and burn the Demon King summoning document?"

I couldn't help it back then. I happened to have run out of firewood. The trees near the village were so resilient that I would need to suffer for an entire day to cut one down. In addition, the freshly cut trees would burn poorly because they wouldn’t dry up. And if a paper like that exists in the first place, won’t the Demon King get summoned again?

"Well, luckily, thanks to you, the advent of the Demon King has been delayed." My aunt smiled lightly and manipulated the holograms to display a portrait of a mask that was half-black and half-white, like half-and-half chicken. Arwen was stunned to see the portrait.

"This guy's code name is Libra, and he's the Demon King worshipper who detected the doppelganger magic you used."

That Half-and-Half Mask was definitely the black magician who aimed for Hillis more than two and a half years ago. At that...