The Borg Queen's vessel was a unique, diamond-shaped starship that was used by the Borg Queen to travel. Little was known about its capabilities or specific purpose, except that the only Federation encounters with this starship design were when the vessel was used to transport the Queen, and thus it could be inferred that it is 'her' ship. The ship was heavily armed and well protected and like all Borg ships, capable of transwarp speeds.
The vessel was present when Species 10026 was assimilated in 2375; Seven of Nine and the Borg Queen were aboard while this assimilation took place. Later, the vessel pursued the Delta Flyer when it escaped from the Unicomplex after rescuing Seven. The vessel was destroyed when the USS Voyager collapsed the transwarp conduit it was traveling in. (VOY: "Dark Frontier")
Background
No designation was formally given for this starship design. According to the video game Star Trek Armada, and many other games, this ship class is referred to as a "Borg diamond," due to its shape and the fact that it follows the naming conventions used in describing the cube and sphere. It remains unknown whether any other ships of this type exist.