* Regal: Royal; majestic; fit for a king.
* Regent: A person who rules a country during the absence or incapacity of the monarch.
* Regicide: The act of killing a king.
* Regime: A system or method of government.
* Region: An area or district, often with shared characteristics.
* Reign: The period during which a sovereign rules.
* Regress: To go back to an earlier or less developed state. (This one doesn't directly relate to "king", but is related to the Latin root.)
* Royal: Relating to a king or queen.
Let me know if you'd like to explore more words with this root or other word origins!