Complex sentence: As a result, Ferdinand became successor to Alfonso's kingdoms of León and Galicia, bringing about a permanent union into what would come to be called the Crown of Castile, wherein the kingdoms continued as administrative entities under the unified rule of a single monarch.
Split into simpler sentences: As a result, Ferdinand became successor to Alfonso's kingdoms of León and Galicia. This resulted in a permanent union into what would come to be called the Crown of Castile, wherein each of the kingdoms continued as an administrative entity under the unified rule of a single monarch.