Question: What is the name of the historical county, later duchy of the Holy Roman Empire, located in the Low Countries that  Adolf van Nieuwenaar worked for in the  Eighty Years' War?
Sentence (from "List of monarchs of Prussia"): The Teutonic Knights were under the leadership of a Grand Master, the last of whom, Albert, converted to Protestantism and secularized the lands, which then became the Duchy of Prussia.
Relevant evidence: No