Their physical appearance was unknown, but Uxbridge described them as "a species of hideous intelligence who knew only aggression and destruction." The Husnock population numbered to fifty billion at the time of their extinction.
It’s learned that the Husnock were non-humanoids. They possessed seven tentacles, four which were used for movement. They had a rear tentacle that was to balance themselves. Their forelimbs ended in trifurcated nimble digits. They also possessed dark blue blood. (TTN novel: Fortune of War)
In 2366, a Husnock warship attacked and destroyed a Federation colony at Delta Rana IV. Unknown to the Husnock, a Douwd was also living on the planet in the guise of Uxbridge. Kevin attempted to divert them away from the colony using his illusions however it only served to anger the Husnock. They finally reached the colony and massacred all the colonists.
In a split-second of anguish over the Husnock attackers killing his wife, the normally pacifist Douwd destroyed every member of the Husnock race with its enormous powers. (TNG episode: "The Survivors", TTN novel: Fortune of War)