The mother kingdom was a kingdom across the Western Ocean from Detina. Several centuries before the Detinan Civil War, explorers from the mother kingdom crossed the Western Ocean and colonized Detina, conquering territory from the blonds and tying them to the land. These explorers established colonies on behalf of the mother kingdom, and ethnic Detinans are descended from the settlers of these colonies. Eventually, the colonies became independent from the mother kingdom and formed the Kingdom of Detina.
The mother kingdom is analogous to Britain.