Both slaves as it was supposed to be

There were some things James didn't want to believe; since Sierra was new to the other side of Revampirina, he thought she misunderstood some things. There was no way a hybrid would exist in Revampirina because the noble vampires hate them. There was no way a condemned would be a vampire and at last, there was no way a condemned would become an Overseer. All humans knew of the castle was only a carriage lavishly built where the Empress moved. No one knew her face.

Nevertheless, Sierra was confident of some things; he called himself a hybrid because he believed it and when he went to Darsoania, he returned completely different with a different air, even his personality took a drastic change. Before, she could tell that he was forcing himself to be cruel but now, it was the new trait his personality took. What happened exactly at Darsoania?