Question: Who published a supplemental sourcebook to the core rules of the second edition of the "Advanced Dungeons & Dragons" designed by an American game designer at a company where he was employed for over fifteen years?
Sentence (from "The Marklands"): The Marklands is a sourcebook for the "Dungeons & Dragons" fantasy role-playing game that describes the realms of Furyondy, Highfolk, Nyrond in the game's "World of Greyhawk" campaign setting.
Relevant evidence: No