Ordo Hermei was founded by twelve of the most powerful wizards of eighth century Europe. Most were former members of the Cult of Mercury, a Roman order of wizard-priests who had served the Republic and Empire during its days of greatness. Others were wizards of Celtic and Germanic mystical traditions.

These twelve wizards were the Founders of the Order and each also founded a Domus ('house') within Ordo Hermei. These Domus are separate traditions, each with its own unique approach to magic. All wizards of the Order, who are called magi, belong to one of these Domus. Each Domus has a founding location, called a Domus Magna, where the leader of the Domus typically resides, and which serves as the cultural center for that Domus.

Ordo Hermei is sub-divided into rough geographical areas called Tribunalia. Each Tribunal is an administrative, judicial, and social unit within the Order. Each magus may have a single registered Sanctum within one Tribunal. Magi may travel between Tribunalia, but may claim only one Sanctum. A Sanctum is a sanctuary, where a magus lives and works his magic. The Sanctum of a magus is often uniquely arranged for the magical methods and personality of a magus.

Ordo Hermei is a large and diverse group of men and women capable of wielding great power. Lex Hermei, the Code of Hermes, is the law that governs this group. The supporters and enforcers of this law are the Quaesitores ("judges"), who are entrusted with a high degree of responsibility in upholding the code, and if necessary, dealing with magi who violate it.

A group as large and wide-spread as Ordo Hermei must also have an effective means of communication to maintain its cohesiveness. Caduceatores of Domus Merceris are specially trained messengers, most of whom are not actual wizards, who carry messages and other items between magi, keeping them informed of events within the far-flung Tribunalia of Ordo Hermei, and facilitating an exchange of ideas and knowledge.

Most magi gather in communities called covenants, which provide the support needed to allow them to pursue their mystical goals. Covenants may take any of a wide variety of forms, from groups of magi in rural estates to a single maga dwelling in a town residence. Most seek to locate themselves within areas of mystical power, called auras, in order to draw on that mystical power for their magic. Magi also seek to control and exploit sources of vis, raw magical power that manifests in a physical form.