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 lashed out in a violent rage, which resulted in the complete annihilation of the entire Husnock species. (TNG episode: "The Survivors", TTN novel: Fortune of War)
The Husnock were organized into a stratified caste system in which the lesser castes pay homage to the higher ones by groveling. The upper castes were known never to allow their limbs to touch the ground. They were a feudalistic society ruled by a king. (TTN novel: Fortune of War)