King Eric was the ruler of the Kalmar Union, from 1396 till 1439. With this respect, he was called King Eric III of Norway, King Eric VII of Denmark, and King Eric XIII of Sweden. Confusing for a king! Therefore, everyone nowadays refer to him as Eric of Pomerania as he finished his life as the Duke of Pomerania. He was born Boguslav, son of the Duke of Pomerania and was related to the royal families of Noway, Denmark and Sweden. He became the heir to the throne of Queen Margaret I of Norway and Denmark at the death of her son Olav and was brought to the court of the Queen in Denmark in 1389. Queen Maragaret I renamed him Erik, on order to sound more "nordic". In September 1389, he became King of Norway, then in 1396, King of Denmark, and finnally in 1397, King of Norway, Denmark and Sweden in Kalmar, establishing what would become the Kalmar Union. But Queen Maragaret I remained in full control of the Kalmar Union until her death in 1412.

As a King, he established Copenhaguen as the capital of Denmark. He implmented the "Sound dues", a tax for ships entering the Baltic Sea through Copenhaguen and constructed the Kronborg Castle at the narrowest part of the sound in order to enforce it. This Castle will become world famous as the place where Shakespeare has set his tragedy Hamlet, Prince of Denmark. It resulted in tension and war with the Hanseatic League, which affected trade in the Baltic and resulted in discontent among the Swede nobility. Swedes resented injustice concerning the way Dans were appointed to lucrative positions in Sweden. It triggered the a great Engelbrekt Rebellion in 1434. Finally, he alienated the Danish nobility as well as he tried to establish Bogislaw IX, Duke of Pomerania, his cousin as the next King for the Union. In response to the refusal by the Danish nobility, he left Copenhaguen for the castle of Visborg in Visby, and was destituted consequently.