Husnock was a planet, located somewhere in the galaxy's Alpha or Beta Quadrants, the homeworld of the Husnock civilization.
In the mirror universe, Husnock came under the control of the Terran Empire some time during the mid-to-late 23rd century.
Under imperial control, the entire planet served as a containment camp for the native and psychotically violent species of the planet, the mirror universe's Husnock. The empire maintained several bases on the planet where scientists used behavior-modification techniques with the intention of disciplining the Husnock and subsequently forcing them to serve the empire's needs as ground troops. (Decipher RPG module: Through a Glass, Darkly)