Sword of Damocles

is the symbol of the Kings. It's made of pure energy which manifests into a gigantic sword that hovers above the King's head whenever they use their powers.

Appearance
The swords each have a unique design and are distinguished from each other by the Aura that pulsates around them. For example, Suoh's can be identified by the Red Aura, while Munakata's sword has Blue Aura pulsating around it.

Danger
When the sword starts to crumble and decay, it's considered dangerous and is a sign that the owner is losing control of his/her power. The sword shows the truest image of its King's condition. When a King has reached its limit, the orb at the center of the sword will stop glowing and the sword will come crashing down, thus killing the owner of the sword. However, this event could be prevented if the dying King is killed before the sword crashes into him, in which case the sword will vaporize and vanish. 

Trivia

 * The sword which appears above the Clan's Kings is influenced from an ancient Greek myth, where a man named Damocles was offered a chance to sit on a throne but with a huge sword hanging above him by a thin thread. The hanging sword became known as The Sword of Damocles ever since.