After war, the number of pure blood devil number decreased and almost went extinct except for 3 clan who did survive. So they are taking humans to increase their numbers in the form of chess pieces. I have only watched the anime and information about Raiser Phoenix's chess pieces was very little. However, it was mentioned that his own sister Ravel Phoenix was a bishop who was revived as demon. What does it mean - isn't she a demon already?
Does that mean Rias Gremory is also servant of higher rank devil, or every pure blood devil are king and they choose their chess pieces/servant except Ravel Phoenix's case where she died and revived as demon as other piece of chess?
And to be chosen as a chess piece, must the person die or not, since the word revived is used. What are the conditions for someone to be chosen the servant? Can it be anyone?