The Duchy of Holstein was established by a Scandinavian monarch under what union?
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.
Magnus IV of Sweden Magnus IV (April or May 1316  – 1 December 1374; Swedish "Magnus Eriksson") was King of Sweden from 1319 to 1364, King of Norway as Magnus VII (including Iceland and Greenland) from 1319 to 1343, and ruler of Scania from 1332 to 1360. By adversaries he has been called "Magnus Smek" (English: "Magnus the Caresser").