Like many popular role-playing environments of the time, Hârn took place in a quasi-medieval setting, on the world of Kèthîra in what could be described in another reality, but described in detail many factors that other systems often neglected. One section of HârnWorld, for instance, laid out a complex model for generating weather patterns that affected the entire planet. Hârn was unusual at its outset for having no defined system to support the story – many early players used Dungeons & Dragons rules systems or RuneQuest models to actually play Hârn campaigns. It wasn't until 1986 that Columbia Games released Hârn's companion rule system, HârnMaster I, which was a leap forward in RPG design in that it calculated the probabilities of various battle, exploration and magic scenarios in a far greater degree of detail than any existing system of the time. The level of detail involved in both modules and statistics made it intimidating for more faint-hearted players. Hârn meta-material was available in the form of CGI's HârnQuest Magazine, released four times annually, which is still published today. At one time Hârn fandom also had a thriving mailing list called HârnLine, also known as the Duffleboard for its moderator Rob Duff.