In the past of Kira's people(otherwise known as the entire human population), a horrible tragedy refered to as "The Ruin" has distroyed everything. A nation that was once very developed and advanced, now has to start from the very beginning.
The "Ruin" song, sung by the singer explains the war-filled history of the past world, from the beginning of time to their present (our future). It starts with the story of creation, leading to the rising and falling of civilizations. This cycle of damage and renewal are similar to the history of prehistoric times, the ruin of the Ice Age, the rise of governments like the Roman Empire, and ending with warfare, conflict and death. The times of ruin, can be related to modern day as the world wars and nuclear extermination with scattered periods of peace and relaxation.
"The Ruin" specifically refers to the predidcted future of humanity. It is fortold that our race and everything on earth will be distroyed because of some great event. Kira's population is some of the people who have survived this horrible wreckage. "The Council of Guardians" are all very old and contain much knowlege. It is possible that these men know very deep secrets into the past, and is why they are treated as royalty. They contain information that will lead their peoples future.
"Ruin.Rebuilding.Ruin again.Regrwoth" Pg.117
"She could see their pattern for growth and now each time the ruin was worse and the rebuilding more difficult." Pg.118