The Kradin were a species in the Delta Quadrant. They were engaged in a war with a race called the Vori, although it is not clear which side started the war, or indeed if either was native to the contested planet.
The Kradin were a large, humanoid species with muscular ridges around the face, and teeth on all sides of their mouth. According to the Vori, they were "beasts" who were capable of a variety of atrocities, including rape and genocide. However, there was no direct evidence for this. In fact, when Commander Chakotay of the USS Voyager was kidnapped by the Vori, the Kradin aided in his rescue.
It is interesting to note that the Kradin claimed that the Vori were the aggressors and accused the Vori of committing the same kinds of atrocities against the Kradin, but again there was little evidence for this. (VOY: "Nemesis")