A masked species who joined the Dominion.
The Breen are a secretive species who are almost never seen without their full-body refrigeration suits and helmets, and their speech is heard only as electronic noise. What they look like underneath is not known to outsiders, and their homeworld is said to be cold, although even that is uncertain. They are feared as raiders and are known for a weapon that drains a ship's power in a single strike.
The Breen were first mentioned in The Next Generation and appeared on screen in Deep Space Nine. Late in the Dominion War, in 2375, they signed a treaty with the Dominion and launched a surprise raid on Earth that damaged Starfleet Headquarters in San Francisco. Their energy-dampening weapon then destroyed an allied fleet at Chin'toka.
Starfleet engineers eventually found a defence against the weapon, and the Breen were among the forces defeated at Cardassia. The Breen returned in the fifth season of Discovery, set in the thirty-second century, where rival Breen factions play a major part.