Complex sentence: In contrast to a traditional RPG gampaign, the campaign rules are set by the campaign management called "The Circle," who also publish the adventure modules, which DMs are required to follow closely.
Split into simpler sentences: In contrast to a traditional RPG gampaign, these rules are not set by a single DM, but rather by the campaign management (called "The Circle"). This management body also publish the adventure modules, which DMs are required to follow closely.