King is a weapon upgrade in Dying Light.
This weapon upgrade will add +2 to Damage, Handling and Durability of a weapon, and is statistically equivalent to the Clicker upgrade. It is the one of the two best upgrades you can find in Dying Light.
Locations to find this upgrade are rare, but can be obtained in very hard locked chests, as well as in The Following DLC Military Chests. A free way to obtain this Upgrade is to play the Dying Light Companion App if you have a smartphone or similar device, and send the "loot" collected by your survivors to your character.