The golden ring is the ring given to Del by Kira. It contained the treasures of the Royal Family of the Larton Kingdom pass down through the generations
Since it looks like the Reviving King is the first Larton Kingdom's King then it was possible that he left some sort of manual about the soul. The Divine Soul Kingdom and the Reviving King are well versed in anything related to the soul!
If Cairo's theory was correct, there should be something related to souls in this ring, right?
However, in the end, Cairo was disappointed. There was no such thing as valuable soul-related manuals. Well, it makes sense. Even if the Reviving King is the first King, he might have only made the Larton Kingdom on a whim
His identity might not even be disclosed and he became an ordinary King. Besides, if the Larton Kingdom really possess such a thing, wouldn't they become another Kingdome specializing in the soul department?