Pure Blood devil and their Chess pieces 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?
I read the light novel, and you got it wrong. It wasn't that only 3 devils survived. Think of it more like they were an endangered species (more than 3), and to increase their number one devil created the chess pieces. When a devil wants a servant, it doesn't limit to just humans, it can be any creature (devils included, i.e. Ravel). For example, a dragon that trains Issei is actually a devil as well. As for someone having to die before they can be a devil is false, it's more like they are just reborn as a devil when they are turn into a servant.

Both Rias and Ruval are pure bloods meaning their parents are devils. In Rias' case, she has no King, but she could if another devil wanted her as their servant (which is the case with Ravel).

Lastly, not all Kings are pure bloods. I'm pretty sure the first season explains that if Issei tries hard enough, he can get his own set of chess pieces making him the King.