Tactical Cube 138 was a Borg tactical cube in service with the Borg in the 24th century.
In 2376, tactical cube 138 was engaged by the starship USS Voyager in which Captain Janeway, Lieutenant Commander Tuvok, and Lieutenant Torres beamed aboard to deploy a nanovirus into the cube's central plexus. As part of the plan, the away team was assimilated into the Collective, while the virus worked to separated the members of Unimatrix Zero from the hive mind.
In the end, the Borg Queen initiated the self-destruct sequence for Tactical Cube 138 in an attempt to kill the Voyager officers. The starship USS Voyager, however, beamed them away just before the cube's destruction. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")