@Kiriha No the emperor of darkness is the emperor of darkness, but that passage was the old man explaining to the knight girl that first, originally there are 7 geniuses in their world, and they are the demon lords.
"The demon lord" that everyone knew was only the emperor of darkness, one of the 7 demon lords. She then says , but record shows that the emperor of darkness came out of a space time distortion( which means he's not from our world)
Then, the old man denies that and says his ancestor(s) heard that from another one of the demon lords that the space time distortion is caused by some kind of ability that let them travel worlds, and that demon lord also told his ancestors a prophecy that he will return there on the 2500th full moon whatever because someone strong will be there, and that demon lord was called "emperor of earth"
edit: made slight wording adjustments in one of those pgs, hopefully it is super super clear now