When he was still the Duke of Duance, he already knew about the existence of demons. However, this information is not from the Annals that Martin Lux just gave him. It was from the documents he was able to gather when he looked for the temple of the goddess that the previous generations buried.
Emperor Ranario was well aware of the transgressions that the past Emperors and the Members of the Seven Families committed to secure the power they held. They waged war against the sorcerers and the people who opposed them. They conquered and slaughtered their way to their rule. Then they burned the books and anything that would expose the truth to everyone else.
It was a sad fact—something that made Ranario dislike the seven families. Now that he was already the emperor, he was trying to look for ways to expose a few parts of the forgotten history. He knew this would affect the other seven families, but to him, some things needed to be publicized.
