Diadem of Maunstraut is the first full-length English fan game made with Sword of Moonlight: The King´s Field Making Tool. It is a first person perspective action-RPG with a dark medieval setting. It is traditional to the King´s Field series only in controls and certain enemies, items and equipment; the story and concepts differ as a fan made game.

The game´s hero will explore a vast world to seek out: powerful weapons, durable equipment, helpful items and potent spells in order to destroy the source of evil throughout the land of Maunstraut. Many powerful evil incarnations, as well as people who may (or may not) help you along the way will be encountered in this long and perilous journey.

The game has a concept similar to the traditional King´s Field games (and other games of the series) as well as its own original soundtrack.