Designated by Starfleet as the Borg Type 03, this class of Borg starship was used by a rogue faction of Borg led by Lore. This unusual design was asymmetrical and totally unlike the previously encountered Borg cube and Borg scout. However, the vessel was still heavily armed and massively armored, and capable of great destructive power.
Only one ship of this type was observed, when in 2369 this group of rogue Borg began attacking Federation outposts and colonies in several outlying sectors.
The ship attacked the research outpost on Ohniaka III, as well as the MS-1 colony, using previously-unknown transwarp conduits to appear suddenly over the target and launch a quick attack. In both cases it confronted the USS Enterprise-D, which responded to the colonies' distress signals. The Enterprise then followed the Borg ship through the conduits to locate their base of operations. (TNG: "Descent")
As numerous Enterprise personnel searched the surface of the unnamed world for the missing Commander Data, a deadly game of cat-and-mouse played out in orbit. The Borg ship attacked, forcing the Enterprise, under the temporary command of Doctor Beverly Crusher, to flee to the edge of the star system. However, Crusher determined to retrieve the remaining crew who had been left behind, and returned to the planet, evading the Borg ship's attack long enough to retrieve most of the remaining personnel. However, the Enterprise delayed long enough that its warp drive was crippled by the Borg ship. Forced to flee again, but this time restricted to impulse speeds, Crusher ordered the Enterprise to activate an experimental metaphasic shield that allowed the ship to enter the nearby star's corona, where the Borg ship could not follow.
The Borg ship held position outside the corona, waiting for the Enterprise to emerge. However, the Enterprise used a particle beam to cause a solar flare that completely engulfed the Borg ship, destroying it. (TNG: "Descent, Part II")
Background
Vessel designation display.
The model for the Borg type 03.
Rogue Borg ship in a pinball machine
This class of Borg ship was designed by Dan Curry and built by Greg Jein. The design of this vessel would later be reused to represent a display for a Borg multi-kinetic neutronic mine in VOY: "Scorpion, Part II".
The "Type 03" designation is visible on a monitor in "Descent, Part II". This classification is presumably because this was the third class of Borg starship encountered by Starfleet.
Apocrypha
This design was designated as an "Assimilator" in the PC game Star Trek: Armada II. This a reference to its in-game special weapon, that can assimilate the crew members of a target and add them to the player's or CPU's own crew reserves.
The ship was used as the centerpiece in the Star Trek: The Next Generation pinball machine.