Tactical Cube 138 was a Borg tactical cube in which Captain Janeway, Commander Tuvok, and Lieutenant Torres deployed a nanovirus into the cube's central plexus. The virus separated the members of Unimatrix Zero from the hive mind. The Borg Queen found out about the virus and destroyed several Borg starships to eliminate the members of Unimatrix Zero.
In the end, the Borg Queen initiated the self-destruct sequence for Tactical Cube 138 in an attempt to kill Janeway, Tuvok, and Torres. The starship USS Voyager, however, beamed them away just before the cube's destruction. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")