Heavily armoured Borg cube used for frontline duty, the setting of Janeway's infiltration in Unimatrix Zero.
The Borg tactical cube was a heavily armed and armoured version of the standard Borg cube, its surface reinforced with additional plating for frontline duty.
In Unimatrix Zero, Janeway, Tuvok and B'Elanna Torres allowed themselves to be assimilated aboard Tactical Cube 138 in order to release a virus that let drones keep the individuality they enjoyed in Unimatrix Zero. The virus allowed liberated drones to break from the Collective and form a resistance against the Borg Queen, and the three officers were rescued by Voyager.