The Subjugation of the Nine Realms was a conflict orchestrated by King Odin Borson of Asgard in an attempt to conquer the Nine Realms and establish an empire.
King Bor lost his life in battle, resulting in his son, Odin, becoming the ruler of Asgard.
Odin later gave birth to his first daughter, Hela, putting under intense training during her childhood.
Hela and her father started to wage war on other kingdoms, as they wanted to control the realms.
Over the centuries, Hela and Odin, as well as Fenris and the Berserkers eventually conquered all the other realms, which embarked the creation of Asgard's empire.