The primary faction of the Pirate Nation to which players may belong.
In the PotBS context, the founder and leader of the Brethren is William Kidd, and they function as the Pirates' National Navy (i.e. the equivalent of the national Navies of Britain, France and Spain).
The Brethren or Brethren of the Coast were a loose coalition of pirates and privateers commonly known as buccaneers and active in the seventeenth and eighteenth centuries in the Atlantic Ocean, Caribbean Sea and Gulf of Mexico.
They were a syndicate of captains with letters of marque and reprisal who regulated their privateering enterprises within the community of privateers and with their outside benefactors. They were primarily private individual merchant mariners of Protestant background usually of English and French origin.
During their heyday when the Thirty Years War was devastating the Protestant communities of France, Germany, and the Netherlands and England was engaged in various conflicts, the privateers of these nationalities were issued letters of marque to raid Catholic French and Spanish shipping and territories.
Based primarily on the island of Tortuga off the coast of Haiti and the city of Port Royal on the island of Jamaica. The original Brethren were mostly French Huguenot and British Protestants, but their ranks were joined by other adventurers of various nationalities including, Spaniards, and even African sailors, as well as escaped slaves and outlaws of various sovereigns.
In keeping with their Protestant and mostly Common Law heritage the Brethren were governed by codes of conduct that favored legislative decision-making, hierarchical command authority, individual rights, and equitable division of revenues.
Henry Morgan is perhaps the most famous member of the Brethren and the one usually noted with codifying its organization. However, following the demographic changes which featured the rise of slave labor in the Caribbean islands, most maritime families moved to the mainland colonies of the future United States or to their home countries. A few, unable to compete effectively with slave labor, enamored of easy riches, or out of angst continued to maintain the Brethren of the Coasts as a purely criminal organization which preyed upon all civilian maritime shipping. This second era of the Brethren began the start of the age of piracy and brigandage which featured the Caribbean until socioeconomic and military changes of the late 18th and early 19th century finally broke its back.
See the Wikipedia article on the Brethren of the Coast