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According to traditions retold by Duarte Lopes, Kongo's ambassador to Rome, and published by Filippo Pigafetta in 1591, Nsundi was formerly an independent small kingdom which was incorporated into Kongo's domains probably in the early to mid-fifteenth century. The province was expanded by its earlier holders east and west along the south bank of the Congo River, and in the late fifteenth century also on the other bank of the river. Portuguese military forces, sent to assist the kings of Kongo in 1491 were deployed to put down a rebellion from sections on both banks of the river.
Later tradition suggested that it was often given to the king's chosen successor to rule, and in fact, the earliest known ruler of Nsundi, Afonso Mvemba a Nzinga, son of Nzinga a Nkuwu, who reigned in the late fifteenth century did become king in 1509. However, the subsequent history of the country does not support the idea that the heir apparent always held this post, and in fact kings came from many different provinces.
Nsundi was a royal province, meaning that it was governed, at the king's pleasure, by an appointed official on a limited term, often three years. When King Álvaro II renamed the provincial nobility along European lines in the late sixteenth century, Nsundi was declared a Duchy. One of the more powerful of its Dukes, Manuel Jordão, served as something of a king-maker in the period 1624-28, before he was humiliated and removed by King Ambrósio I in 1628.