Under what union did the king who created Duchy of Holstein serve?
Duchy of Holstein: The Duchy of Holstein (German: "Herzogtum Holstein" , Danish: "Hertugdømmet Holsten" ) was the northernmost state of the Holy Roman Empire, located in the present German state of Schleswig-Holstein. Duchy of Holstein:  It was established when King Christian I of Denmark had his County of Holstein-Rendsburg elevated to a duchy by Emperor Frederick III in 1474. Christian I of Denmark: Christian I (February 1426 – 21 May 1481) was a Scandinavian monarch under the Kalmar Union.
the Kalmar Union