The King is the player with the highest number of bones in-game. When a player become the King and appear at the top of the In-Game Leaderboard, the player will hear a voice saying: "You are the King", as well as getting a crown.
The king status can be lost in 2 ways, when someone passes the top player in bone count, or when the king dies and loses all his bones. When the player loses the status of a king they will hear a voice saying: "You lost the lead".
In the desert mode where the larger the number of bones the king has, the larger the player becomes, this increases the hitbox of the king and therefore makes the king easier to kill. However, this also increase's the kings reach so this should be taken advantage of when possible. Other players can see the king's location via the player radar.
In earlier versions of the game when the player became king, a small skull icon appeared above his head. However, this was removed with the bone count.