Rethem is a kingdom module set in HârnWorld detailing the kingdom of Rethem in western Hârn. Rethem is a chaotic and unstable young kingdom, where the dark religions of Morgath and Agrik hold great power.

The module has information on the history, organization and current events of the kingdom. It also contains detailed descriptions of the castles of Shostim and Tormau and the ruined fortress of Kustan. Included are articles detailing the barbarian tribes of Equani, Kubora and Urdu. Rethem also gives information on Lia-Kavair, the "Thieves' Guild" of Hârn.