Question: Under what union did the king who created Duchy of Holstein serve?

Context paragraphs:
Treaty of Ribe: The Treaty of Ribe (Danish: "Ribe-brevet" meaning The Ribe letter; German: "Vertrag von Ripen" ) was a proclamation at Ribe made by King Christian I of Denmark to a number of Holsatian nobles enabling himself to become Count of Holstein and regain control of Denmark's lost Duchy of Schleswig (Danish: "Sønderjylland", i.e. "South Jutland").  The most famous line of the proclamation was that the Danish Duchy of Schleswig and the County of Holstein within the Holy Roman Empire, should now be, in the original Middle Low German language, "Up Ewig Ungedeelt", or "Forever Undivided".  This was to assume great importance as the slogan of German nationalists in the struggles of the 19th century, under completely different circumstances.

Province of Schleswig-Holstein: The Province of Schleswig-Holstein (German: "Provinz Schleswig-Holstein" , ] ) was a province of the Kingdom of Prussia and the Free State of Prussia from 1868 to 1946.  It was created from the Duchies of Schleswig and Holstein, which had been conquered by Prussia and the Austrian Empire from Denmark in the Second War of Schleswig in 1864.  Following the Austro-Prussian War in 1866, which ended in Austrian defeat, Schleswig and Holstein were annexed by Prussia.  The province was created in 1868 and also included the Duchy of Lauenburg since 1876.

List of consorts of Schleswig and Holstein: The Duchesses of Schleswig-Holstein were the consorts of the rulers of Schleswig-Holstein and the separate states of Schleswig and Holstein, before that, the two duchies of Schleswig and Holstein.  This article would focus more on the Duchess consorts of Schleswig and Holstein, Schleswig-Holstein (in pretense), and the many branches of the Schleswig-Holstein duchy created by the Danish king for his relatives.

Sejm of the Duchy of Warsaw: Sejm of the Duchy of Warsaw (Polish: "Sejm Księstwa Warszawskiego" ) was the parliament of the Duchy of Warsaw.  It was created in 1807 by Napoleon, who granted a new constitution to the recently created Duchy.  It had limited competences, including having no legislative initiative.  It met three times: for regular sessions in 1809 and 1811, and for an extraordinary session in 1812.  In the history of Polish parliament, it succeeded the Sejm of the Polish-Lithuanian Commonwealth and was followed by the Sejm of the Congress Poland.

Schleswig-Holstein-Sonderburg-Plön: The Duchy of Schleswig-Holstein-Sonderburg-Plön (German: "Herzogtum Schleswig-Holstein-Sonderburg-Plön" ), also "Schleswig-Holstein-Plön", "Holstein-Plön" or just "Duchy of Plön", was a small sub-duchy ("Teilherzogtum") created by the physical division of the Duchy of Schleswig-Holstein-Sonderburg.  Today, its remaining significance is primarily the building of Plön Castle.  The Duchy of Plön was not a territorial dukedom in its own right, but a sub-division within the state structure of the duchies of Schleswig and Holstein.  The scattered territorial dominion lay mostly in the southeast part of present-day German state of Schleswig-Holstein.

Schleswig-Holstein-Sonderburg: Schleswig-Holstein-Sonderburg was the name of a branch line of the House of Oldenburg as well as the name of their land.  It existed from 1564 until 1668 and was a titular duchy under the King of Denmark, rather than a true territorial dukedom in its own right.  The seat of the duke was Sonderburg.  Parts of the domain were located in Denmark (in the Duchy of Schleswig), mainly on the islands of Als and Ærø and around Glücksburg, whilst other lands were part of the Holy Roman Empire (in the Duchy of Holstein), including the "Ämter" of Plön, Ahrensbök, and Reinfeld.  As a result of various inheritance arrangements it fragmented into numerous small territories which were eventually absorbed into Greater Denmark in the 18th century.

Christian I of Denmark: Christian I (February 1426 – 21 May 1481) was a Scandinavian monarch under the Kalmar Union.  He was King of Denmark (1448–1481), Norway (1450–1481) and Sweden (1457–1464).  From 1460 to 1481, he was also Duke of Schleswig (within Denmark) and Count (after 1474, Duke) of Holstein (within the Holy Roman Empire).  He was the first Danish monarch of the House of Oldenburg.

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.  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.  Holstein was ruled jointly with the Duchy of Schleswig by members of the Danish House of Oldenburg for its entire existence.

List of monarchs of Prussia: The monarchs of Prussia were members of the House of Hohenzollern who were the hereditary rulers of the former German state of Prussia from its founding in 1525 as the Duchy of Prussia.  The Duchy had evolved out of the Teutonic Order, a Roman Catholic crusader state and theocracy located along the eastern coast of the Baltic Sea.  The Teutonic Knights were under the leadership of a Grand Master, the last of whom, Albert, converted to Protestantism and secularized the lands, which then became the Duchy of Prussia.  The Duchy was initially a vassal of the Kingdom of Poland, as a result of the terms of the Prussian Homage whereby Albert was granted the Duchy as part of the terms of peace following the Prussian War.  When the main line of Prussian Hohenzollerns died out in 1618, the Duchy passed to a different branch of the family, who also reigned as Electors of Brandenburg in the Holy Roman Empire.  While still nominally two different territories, Prussia under the suzerainty of Poland and Brandenburg under the suzerainty of the Holy Roman Empire, the two states are known together historiographically as Brandenburg-Prussia.  Following the Second Northern War, a series of treaties freed the Duchy of Prussia from any vassalage to any other state, making it a fully sovereign Duchy in its own right.  This complex situation (where the Hohenzollern ruler of the independent Duchy of Prussia was also a subject of the Holy Roman Emperor as Elector of Brandenburg) laid the eventual groundwork for the establishment of the Kingdom of Prussia in 1701.  For diplomatic reasons, the rulers of the state were known as the King in Prussia from 1701 to 1772; largely because they still owed fealty to the Emperor as Electors of Brandenburg, the "King in Prussia" title (as opposed to "King of Prussia") avoided offending the Emperor.  As the Prussian state grew through several wars and diplomatic moves throughout the 18th century, it became apparent that Prussia had become a Great Power that did not need to submit meekly to the Holy Roman Empire.  By 1772, the pretense was dropped, and the style "King of Prussia" was adopted.  Thus it remained until 1871, when in the aftermath of the Franco-Prussian War, the King of Prussia Wilhelm I was crowned German Emperor.  From that point forward, though the Kingdom of Prussia retained its status as a constituent state of the German Empire, all remaining Kings of Prussia also served as German Emperor, and that title took precedence.

Prussia (region): Prussia (Old Prussian: "Prūsa", German: "Preußen" , Lithuanian: "Prūsija" , Polish: "Prusy" ) is a historical region stretching from Gdańsk Bay to the end of Curonian Spit on the southeastern coast of the Baltic Sea, and extending inland as far as Masuria.  The territory and inhabitants were described by Tacitus in Germania in AD 98, where Suebi, Goths and other Germanic people lived on both sides of the Vistula River, adjacent to Aesti (further east).  About 800–900 years later the Aesti were named Old Prussians, who since AD 997 repeatedly successfully defended against take-over attempt by the newly created Duchy of the Polans.  The territory of the Old Prussians and neighboring Couronians and Livonians was unified politically in the 1230s as the Teutonic Order State.  Prussia was politically divided in the period 1466-1772, with western Prussia under protection of the crown of Poland-Lithuania and eastern Prussia a Polish fief until 1660.  The unity of both parts of Prussia remained preserved by retaining its borders, citizenship and authonomy until western and eastern Prussia were also politically re-united under the Kingdom of Prussia.  It is famous for many lakes, as well as forests and hills.  Since the military conquest by the Soviet Army in 1945 and the expulsion of the inhabitants it divided between northern Poland (most of the Warmian-Masurian Voivodeship), Russia's Kaliningrad exclave, and southwestern Lithuania (Klaipėda Region).  The former German state of Prussia (1701–1947) derived its name from the region.

Supporting paragraphs: Duchy of Holstein Christian I of Denmark
Answer: the Kalmar Union