Complex sentence: The Duchy of Brabant was initially a feudal elevation of the landgraviat, but its name was applied to the entire country under control of the Dukes of Brabant from the 13th century on.
Split into simpler sentences: The Duchy of Brabant is initially a feudal elevation of the already existing title of Landgrave of Brabant. Nevertheless, its name was lateron applied to the entire country under control of the Dukes pf Brabant from the 13th century on.