The Onyx Task Force are a special group of knights serving under the current king; Charioce XVII.
The Onyx Knights are part of a task force who swear loyalty to the King Charioce XVII, and did not exist prior to Charioce XVII's rule. 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 the green stones which grant them power.
In order to become an Onyx Knight you must place the magical green stone into your chest. By doing so you allow the stones to drain your life force as the cost of obtaining power. Other side effects include 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. As a result, an Onyx Knight will gradually die a slow and painful death.
The Onyx Knights are extremely loyal to their cause, to the point of going behind their king's back to get things done (such as hiring the Dragon Assassin to take out Nina so Charioce XVII wouldn't lose focus of his goal). This isn't to say that they would go as far to harm their king but rather remind him of 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 takes priority in even above their own lives.
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.
Another weakness of the Onyx Knights is El, who possesses the ability to disable the green stones used to empower the Onyx Knights. Because of this, Charioce XVII spent two years tracking and hunting down El, desiring him either dead or alive. The Onyx Knights themselves regarded El as a major threat. Nina Drango is also capable of harming the Onyx Knights.
Their weakness is mainly derived from their abilities being focused on slaying gods and demons but not humans. El and Nina, both being half-bloods, are able to defeat the Onyx Knights for this reason.