The Master Spirits refer to a group of ten guardian spirits that are the current keepers of their official elements. They are chosen by the Beast King to embody one of the nine elements and are tasked with later testing the next Beast King.
Azmyth is the Master Spirit of Energy and the former Beast King.
The Master Spirits were originally each chosen by Azmyth to embody one of the nine elements of darkness, earth, fire, lightning, wood, ice, water, wind and light. They are not born guardian spirits, but rather transformed into one after being selected. Later on, they are tasked with testing Azmyth's intended successor to determine whether he is worthy of becoming the Beast King. When they are defeated, they pass on the powers to the successor (though they still retain their abilities and powers) and begin to serve them.
The previous generation of Master Spirits is called the Origin Spirits.