You've just cleared the Moathouse near Hommlet. In gratitude (and in fear over the potential of evil cults in the area) the Viscount of Verbobonc knights you and grants you the Moat House and lands 3 miles in each direction as your fiefdom.

If I was really developing this, I would introduce the Viscount's Cousin, who's ancestor originally built the moathouse. He vastly prefers his townhouse in Verbobonc and has no interest in living in the "rustic glory" of your new fiefdom.

He does however share with you an architectural sketch of the original moat house construction, pointing out a trap door to stairs under what is now the north wall and offers fine payment and glory for any relics of his family that can be retrieved.