The King of the Demon Realm (魔界の王 Makai no ō, "King of the Demon World"), often just referred to as the Demon King, is an entity who exists in the Demon Realm, acting as a sort of equivalent to the Kai.
The King of the Demon Realm title was given more detail in the second Dragon Ball: Super Exciting Guide, which was released in 2009. Occasionally, when a Core Person with an evil heart is born, the evil Core Person falls under the Demon Realm King.
Dabura and Mira have both been referred to as King of the Demon Realm. Fu is referred to as the rightful successor to his genetic father and uncle. Dabura and Mira also have a fusion named Damira. Dabura can also fuse with Demon King Piccolo to become Demon King Daccolo. Additionally, Mirayo in Dragon Ball Fusions is referred to as a former queen of the Demon World.
Daizenshuu 7 refers to Demon King Dabura as being the absolute ruler who commands all of the Demon Realm, however later media introduced entities who outrank the King of the Demon Realm: the Demon Supreme Kai, Master of the Demon World and Dark King.