Breens

The Breen are a reclusive, powerful, and warlike humanoid race, native to the planet Breen in the Alpha Quadrant. Shrouded in mystery, the Breen were one of the most underestimated races inhabiting the Alpha Quadrant. 
Historically, the Klingons were among the first to discover the consequences of underestimating the Breen. During the Klingon Second Empire, Chancellor Mow'ga ordered an entire fleet of Klingon warships to invade and conquer the Breen homeworld. The fleet never returned and was never heard from again. 
Even by the 24th century, much was still unknown about the Breen and their otherwise politically nonaligned government, known as the Breen Confederacy. The Federation, however, had limited knowledge of the Breen and was aware of Breen outposts located near a black cluster in 2368. In 2370, the Breen participated in a palio held at Deep Space 3, during which the Ferengi attempted to bribe the Breen pilot into throwing the race. In 2373, the Breen settlements on Portas V near the Demilitarized Zone dealt with the Maquis, supplying them with cold-storage units. It was noted by Ezri Dax in 2375, when captured by the Breen on a planet in the Goralis system, that they were a long way from Breen space.
Other species like the Ferengi had developed closer ties with the Breen by the late 24th century, as both species conducted trade negotiations in 2373. 
Breen privateers during the 24th century would often conduct indiscriminate raids against other Alpha Quadrant species. This includes the Breen attack and capture of the Cardassian transport Ravinok in 2366. The survivors of that attack were used for slave labor in the Breen operated dilithium mines on Dozaria. In 2368, Starfleet feared the Breen might have attacked the science vessel SS Vico. Breen privateers also raided the Bajoran colony of Free Haven in 2372; they were, however, successfully driven away by the USS Defiant.
Alliance with the Dominion In 2375, the Breen moved from a policy of low-level hostilities toward other major powers to one of open warfare when they allied themselves with the Dominion. They were then allowed access to the Dominion military database. According to the terms of the alliance, the Breen Confederacy received several planets in the Cardassian Union as compensation for joining the war.
Shortly after joining with the Dominion, the Breen attacked Starfleet Headquarters on Earth in the raid on San Francisco. A short time later they assisted Dominion forces in breaking the lines at Chin'toka.
=== Physiology ===
Breen physiology is among the most exotic for humanoid lifeforms, which may be due, in part, to their homeworld's climate. The Breen have no blood or other liquid circulatory system. The Breen brain is structured into four lobes; this protects them from the probing of some empathic species, such as the Betazoids, who are unable to detect the thoughts or emotions of the Breen. 
Breen appear physically more resilient than most humanoids. When Worf struck a Breen, the Breen proceeded to incapacitate Worf with a neural truncheon, seemingly oblivious to the assault. It is possible this toughness was due, in part, to the Breen's armor rather than its physiology.
The Breen constantly wear refrigeration suits in order to maintain a cold temperature like that on their homeworld (although Weyoun stated that the Breen homeworld is not a cold wasteland). According to Worf, no outsider had ever seen what a Breen looked like under their refrigeration suits and lived. Even after joining the Dominion, their appearance was unknown to their new allies. Weyoun couldn't help but wonder what they looked like.