The Southern Kingdoms were the multitude of kingdoms and principalities that ruled over the land that would become the Empire during the Third Age. Many of the noble houses of the Empire are direct descendants of the ruling and minor houses that arose during the time of the Southern Kingdoms.
Eramus the Conqueror accepted the honorable surrender of many houses and listened to his advisers, striving to incorporate the once ruling houses into the leadership of the Empire. Interweaving the fate of the Empire with the noble houses, he sought to bring stability by giving former kings military and political power, through appointments within the Imperial Guard and the formation of the the Grand Council.
This arrangement lasted for several centuries, before the War of the Throne.