The primary vessel used by the Borg Collective, the Cube is a colossal starship.
During encounters with the Borg in the 2360's and 2370's, the Federation found that Borg Cube to be a nearly indestructible enemy. The first Borg incursion into Federation territories left a fleet of 39 starships derelict or destroyed. Even after being given time to develop new anti-Borg technologies Starfleet was still ill prepared for the second Borg incursion in their territories some years later.
A single Borg cube is capable of launching the invasion and assimilation of an entirely planetary culture. Such ships are enormous in size and carry thousands of Borg drones that can act as warriors and builders for the Collective in their search for technological excellence.
It is a frightening proposition for any race to take on a Borg Cube. In any case, entire fleets must be sacrificed in the destruction of a single one of these ships.