Silver Blood

Both the royal family and the guard have ancestries that date back centuries. Myth and legend tells that while the Royals have blood as calm and dignified silver as the full silver moon, the guard have blood as impenetrable and black as the stone they are aptly named after.
In this kingdom, the only thing that matters is blood. People are divided neatly, and according to their bloodlines. The systematic division of people is, at first glance, simple. The Royals, who live in the central palace, a heavily guarded fortress. The Obsidian guard, who mostly dwell in the mountains training. And everybody else, scattered amongst several rundown villages in the kingdom.
Blood is a constitution in this kingdom. It is blood that governs above all else, as it decrees law and order. In that Kingdom, blood is thicker than water, than flesh, than bone. And it is blood that ties the guard to the Royal family.
The Obsidian guard were created centuries ago to profit the Royals. Folklore told that the Obsidians were birthed from the mountain side, eyes and hearts as pitch black and hard as obsidian stone. Children were taught that Obsidians were created from one drop of royal blood, dropped over a piece of obsidian heart-stone. As history told, the Royals gave the Obsidians life. And the Obsidian lives would be used to protect the Royals. Black blood protecting silver.