Arthur knew this because his memories of that existence that had long returned to him.
He remembers being the Mortal Emperor, remembered the power he had wielded, and most importantly, he remembered exactly where he had hidden his treasures in this secret realm during that lifetime.
The inheritance that countless powers were searching for blindly?
Arthur could walk straight to it.
Every hidden chamber, every concealed vault, every secret location where valuable items had been stored over one hundred thousand years ago - he knew them all.
The reason why he had been able to set things up so that nobody had been able to acquire the Mortal Emperor's inheritance since that time, over one hundred thousand years ago, was because he had originally made a deal with the guardian spirits of the Martial Emperor Secret Realm.
These guardian spirits were ancient beings, entities that predated even his past life as the Mortal Emperor.
