From UFStarfleet LCARS
Native to the Alpha Quadrant, the Breen are an extremely aggressive species with a loosely organized government known as the Breen Confederacy. Known for their low level hostilities against other races, the Breen escalated their aggression when they allied with the Dominion during the Dominion War, although they currently mostly abide by the Treaty of Bajor, signed in 2375, that ended the conflict. Breen privateers continue with their raids against other species, and their reach has extended into the Delta Quadrant. Very little is known about Breen physiology. The Breen have a brain which is uniquely separated into 4 lobes, which has rendered them immune to the probing of empathic species, and it is known they have no blood or any other form of liquid circulatory system. They are stronger than most other humanoids, even able to shrug off the attack of a Klingon. Their actual appearance is unknown, as the Breen wear full refrigeration suits to maintain cooler temperatures to match their homeworld conditions. There are no records of anyone actually seeing a Breen outside of their suit.
The Breen have been a presence in the Alpha Quadrant since before the time of Federation exploration, and there are records that Chancellor Mow’ga, leader of the Klingon Second Empire, attempted but failed to conquer the Breen homeworld. The Breen were first encountered in the Delta Quadrant in 2383 by then Captain Chase Quinnell, CO of the USS Redeemer-A, while enroute to D’Noris, when they responded to a Klingon distress call, successfully engaging and repelling a combined Breen/Romulan fleet.
Not much is known, outside of their political structure and that they tend to have children (by human standards) at an extremely young age.
The Breen are equal to the Federation in technology, with a focus on weapons, one of their most effective being an energy dampening weapon that was able to drain all the main power systems of a starship, giving them total tactical advantage until a defense was finally devised.
USS Redeemer-A archives:
Summarized by Fender Sparta, Stardate 101123