Shin-En: The Abyss. It's a world below our own, one with a civilization that holds technology that goes past our imagination. That includes weaponry and the "military" force of Shin-En; Mecka-Kikai's. They are past criminals or Dorei's who've been made into mental vegetables and merciless killers that enforce the laws of Shin-En.
Next thing to know about Shin-En; there is no "human," but "Dorei" and "Kurai." Dorei's are a peaceful race, lead by a Monarch governement and have the gift of eternal life the moment they reach maturity: or become sixteen. The Kurai, on the other hand are a more of a Fuedalist Government with Five Houses that control all; from technology advancements to architecture and the fine arts. Another thing about Kurai's; they hold special abilities unique to each holder that eventually drains away all of the Kurai's life force if not for one exception; they feed off the life of a Dorei before they become fifteen.
Thirty years ago, this discovery caused a war. Thirty years ago, the Dorei King and Queen were massacred, along with their eldest daughter Evelyn, and their youngest, Rikka, leaving only Sorana alive.
It's now thirty years later and the Kurai are thriving. They have all the power they can get, and all the life they need through Dorei slave trafficking - but that doesn't mean all Dorei's have lost hope.