Overview
The Onyx Knights are a group of knights who swear loyalty to King Charioce XVII. They work as his personal guard that perform duties from body guarding his majesty to taking out major threats. They are specially trained to take out gods and demons with the help of a mysterious power. In order to become a onyx knight you must place a magical green stone into your chest which is their source of power. However, there are major side effects of using these stones. As seen in episode 21, the use of these stones can cause the user's life force to drain away.The other side effects are the formation of bruise like spots all over the body with thick purplish black veins that pop out out and the coughing up of a tar like substance. Despite how powerful the Onyx knights look inside their armor they are literally dying underneath.
Role
Through out the show the show, the Onyx Knights are seen more than just a personal body guards but rather a force committed to their kings goals.
Loyalty
Despite their ruthlessness, the onyx knights are a very loyal to their cause. Even to the point of going behind their kings back to get things done such as hiring a assassin to take out Nina so that Charioce wouldn't loose focus on his goal. This isn't to say that they would go as far to harm their king but rather remind him what their mission is. There connection with their king is rooted in some deep unknown past but it seems their loyalty is derived from their goals that they share which they take priority in even above their own lives.
They are very prominent in protecting his majesty even to the point of risking their lives for his safety.There is a huge amount of trust that Charioce puts into the onyx knight to protect him even to the point of not flinching when an enemy is rushing at him, allowing the onyx knights to stop an attack at the last minute.
However, the pinnacle of loyalty they show is the sacrifice of their bodies in order to power the weapon "Dromos" which their king shares their burden.
History
The Onyx knights came around the time Charioce XVII came into power. They are not a faction of knights of a previous king.