The Brethren were a race of beings first encountered by the Federation in the late 24th century. The D'myurj employed them as foot soldiers.

Their superheated forms were protected by a large suit of armor with built-in pulse weapon technology that they could activate from the palms of their hands. They wore helmets with infrared sensor technology in lieu of normal vision. A small vent on the sides of these helmets allowed them to regulate their body temperature and servesdas their one weakness: anything blocking that vent incapacitated one of the Brethren and soon killed it.

The Brethren first attacked the starship USS Trident, which sustained heavy casualties before the USS Excalibur rescued them. Soon after, Selar and Soleta infiltrated the Brethren base on planet AF1963 to rescue Robin Lefler's son, Cwansi. The operation was successful, though it cost Selar her life. (NF novel: Treason)

Weeks later, the Brethren attacked Mackenzie Calhoun's homeworld of Xenex as part of a plot to eliminate Calhoun and the Excalibur. Calhoun and the Xenexians eventually defeated the forces there, and Calhoun escaped on a Brethren ship. However, the Brethren retuned to Xenex and destroyed most of the population of the planet. (NF novel: Blind Man's Bluff)