Eston's heart was racing even faster than it was before as soon as he heard the words "Dark Magus." Everyone in Alterian knew of the person.
He was the most famous and infamous person in all of Alterian. The reason why his heart was racing was because he believed it.
If Raze had simply come out and claimed that he was the Dark Magus, then Eston would have thought nothing of it, even if he was a mage that used Dark Magic.
It wasn't like he was the only one; there were plenty of criminals that had turned to using Dark Magic, some even trying to copy the fame of the Dark Magus.
But it was what they had been through. With Eston being who he was, being as strong as he was, the number of people that could beat him, he could probably only count them on both of his hands.
Narrow that down to the number of people that knew Dark Magic that could beat him, then in his mind, it was only one.