Brethren

The Bretheren are a race of beings first encountered by the Federation in the late 24th century. The D'myurj employ them as foot soldiers. Their superheated forms are protected by a large suit of armor with built-in pulse weapon technology that they can activate from the palms of their hands. They wear helmets with infrared sensor technology in lieu of normal vision. A small vent on the sides of these helmets allows them to regulate their body temperature and serves as their one weakness: Anything getting into that vent incapacitates a Bretheren and eventually kills it.

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

Weeks later, the Bretheren 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 Bretheren ship. However, the Bretheren retuned to Xenex and destroyed most of the population of the planet.