Breen
A humanoid species from a cold planet who operate in a politically non-aligned manner and are reputed to be aggressive.
While the Federation has some knowledge of their ways, Betazoids are unable to detect the Breen empathically, and the Breen's use of cloaking technologies coupled with powerful disrupter-style weapons makes them both formidable and inaccessible to the Federation. The disrupter-type, close-range damage done to the USS Vico in the Black Cluster is consistent with their battle tactics and their level of technology meaning that the Federation has some knowledge of their ways. But a motive for the Breen to be inside the cluster is puzzling.
A Breen pilot was sent to the palio hosted by Deep Space Three, accused of being bribed by Ferengi to throw the race. They have Type III disrupters like both the Romulans and Klingons, but would not kill all victims.
