The Borg Fusion Cubes are created from cubes constructed by the Borg Advanced Shipyard, and with research conducted by the Borg Technology assimilator. The cubes can be either unarmoured or have armour plate. Eight of each type merge to form either a Fusion Cube, formed from unarmoured cubes, or eight Tactical cubes, which are larger cubes that are armoured, form into a Tactical Fusion Cube. These can use three enemy Special weapons, if it is the unarmoured fusion cube, can use four Special weapons, if it is the armoured Tactical Fusion Cube. It was a Fusion cube that came out of a Borg Transwarp hub, startling Starfleet Command.
The Fusion cube is the size of a LARGE asteroid/small planetoid,like Earth's Moon while the Tactical Fusion cube is the size of Mars.