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. This armor can get extremely hot, hot enough to melt the skin off the bones of anyone who touches it. 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 served as their one weakness: anything blocking that vent incapacitated one of the Brethren and soon killed it.
Brethren were not believers in assigned rank, since they felt that all were equal. However, there were those individuals who, by dint of their personalities, became natural leaders and were simply recognized as such without receiving a separate designation.
Their ships (which were blocky and utilitarian) had no navigator or pilot; all duties aboard the ships were handled by easily controlled computers. It was the philosophy of the Brethren that such duties were best left to machines since it allowed the Brethren's time to be open for matters of far greater consequence, such as fighting, killing, and proving their superior strength by fighting and killing.
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 returned to Xenex and destroyed most of the population of the planet. (NF novel: Blind Man's Bluff)