(covers information from several alternate timelines)
In early 2372, the IKS Negh'Var was announced as the new flagship of the Klingon Imperial Fleet. Commanded by a changeling posing as General Martok, he led the Klingon Empire's invasion of the Cardassian Union around stardate 49011. She thereafter fought an action at Deep Space 9, where she was able to breach the station's shields long enough to transport several troops aboard. (DS9: "The Way of the Warrior")
In the mirror universe, the Klingon-Cardassian Alliance utilized the same design, referred to simply as a cruiser, in the construction of the Regent's flagship. (DS9: "Shattered Mirror", "The Emperor's New Cloak")
In an alternate future around 2395, the USS Pasteur was attacked by two of these attack cruisers while in the Devron system. The crew of the Pasteur was saved by the USS Enterprise-D, after destroying one of the Klingon vessels. (TNG: "All Good Things...")
The outboard plan of the Negh'Var warship design incorporated the same basic shape and classic lineage dating back over two hundred years; indeed, sharing a strong resemblance to the era's Vor'cha-class attack cruiser.
The bridge module was separated from the main body by a thick neck that flared towards the aft of the ship into the forward-swept wings, which were reinforced by raised supports and contained the ship's embedded warp nacelles. Contained within these wings were the ship's impulse engines. Below the primary hull were two weapons pods. A triangular superstructure was located on the dorsal section of the ship. The ship was also outfitted with several spikes, located on the aft of the triangular superstructure and warp nacelles. (DS9: "The Way of the Warrior", etc.)
At least two design variations were conceived for this class found in alternate timelines.
In the first variant, seen in Q's anti-time future, occurring during the late 24th century, the Negh'Var design the ventral weapon pods were removed, additional "spike"- or "antennae"-like structures were placed on the bow of the ship and on the forward part of the nacelle tips, and two "fin" or "fang"-like structures protruded from the ventral section of the bow on the below bridge module. (TNG: "All Good Things...")
The second variant, seen in an alternate 2404 timeline, shared components of both the original and first variant designs, possessing the former's weapons pod and the latter's "spikes" and "fins". (VOY: "Endgame")
Heavily armed, the Negh'Var warship were equipped with several disruptor emplacements. The original design had an emplacement located at the tips of the pods located on the underside of the ship's belly. This class was also equipped with a cloaking device. (DS9: "The Way of the Warrior", "By Inferno's Light")
In one possible future timeline variant, this class could easily outgun an Olympic-class medical ship, yet were heavily outgunned themselves by the Galaxy-class counterpart of the era. This design had disruptor placements aft tips of the primary hull, two outboard disruptors at the front of both wing tips (fore of nacelles), a forward emitter located below the main bridge and several emitters located along the forward baseline of the ship and one on the ventral center of the ship. This variant was also equipped with a cloaking device. (TNG: "All Good Things...")
In a second possible future timeline variant, in 2404 was armed with directed energy weapons that fired nadion discharges. This weapons fire was essentially ineffective against Starfleet ablative generator-deployed hull armor. This class was also relatively matched by a refitted Nova-class starship of the era. (VOY: "Endgame")
In the mirror universe, this design was described by Intendant Kira Nerys as being "quick and powerful", however, were not without their weaknesses; specifically, their targeting systems could be easily fooled. Additionally, the ship proved to be unable to lock onto an enemy vessel attacking from close proximity. (DS9: "Shattered Mirror", "The Emperor's New Cloak") This class was not initially equipped with a cloaking device, but briefly acquired one in 2375 after one was stolen by Quark and Rom from the IKS Rotarran, a prime universe Klingon Bird-of-Prey. Rom was successful in installing the cloak, but in doing so performed an act of sabotage, which resulted in the loss of all primary systems on the ship coinciding with the disengaging of the cloak. (DS9: "The Emperor's New Cloak")
Ships of the class
The designation for this class can be read briefly on an okudagram appearing in several episodes of DS9 and VOY, which reads "Negh'Var warship". In "All Good Things..." it was simply referred to as an attack cruiser.
The Negh'Var was a modification of the original "future Klingon ship" studio model used in "All Good Things...", which was designed by Rick Sternbach during March 1994 as a modified version of the Vor'cha-class model, and built by Gregory Jein.  The molds of the Vor'cha model were modified and used for construction of the new ship. Its ancestry can be seen in the center part of the hull which is identical to to the Vor'cha.
According to Sternbach:
"An alternate timeline upgrade of the Vor'cHa was called for in the story, and so some proposed modifications were made to the original attack cruiser sketch, suggesting new hardware bits that could be attached to the existing filming miniature. These mods were drawn up in March of 1994, with the same basic styling, hopefully looking different enough to say it was a new design. The word came though that the producers wanted a somewhat more radical take on the cruiser, so another sketch followed (our producers always liked lots of options). The ship model that would eventually be called the Negh'Var took some of the add-ons for the Vor'cHa and made them integral to the design, with some new proportions. Definitely still Klingon, but different enough to see it wasn't a Vor'cHa with some bits stuck on. The wings platform was reversed, bending forward to give a hint of a bird-like shape, the engines were clipped slightly and embedded within the wings, and the "canards" were blended into the nose area. A new main bridge, an ominous snake head and "fang" antennae, and a new disruptor cannon rounded off the main changes. This particular ship didn't have a class name, as far as I can recall, being a vessel that ended up not existing in the "fixed" timeline. How this imaginary cruiser ended up becoming the Negh'Var, I don't have any direct knowledge, because the miniature was modified by our good VFX folks and model makers and didn't really require my involvement, and you can see the results here in this pictorial. Like most ship designs I got to contribute, these two were an absolute ton of fun."
Sternbach envisioned his design outfitted with a clamped-on auxiliary craft, attached to the bridge section, "I suppose the "daughtercraft" was an extension of the thing I did with the Vor'cHa with its main disruptor cannon section, in that it was detachable in a fight if damaged. I was influenced by the movie The Beast, which Richard James worked on, where one of the Russian tank guys said, "Out of commission, become a pillbox. Out of ammo, become a bunker. Out of time, become heroes." Figured it would work for the Klingons. It wasn't that the little ship was there to let them run away, but more to give them more options to stay in a fight. All a matter of what you do with the gear you've got."
A separate port wingtip section (along with other pieces) was built, cast from the molds of the master model, and used to show that piece being blown off in the episode. In this finish a copy of the model, cast from the original molds has been on tour in 1996 among others at the LA Star Trek Convention.
Although originally intended to make an one-time appearance, the appeal of the model was such that it was decided to bring it back as the Negh'Var for DS9: "The Way of the Warrior". Returning in late May 1995 from the opening leg of the Star Trek: The Exhibition tour in Scotland, Effects Supervisor Gary Hutzel was dismayed when he discovered that the model was severely damaged: "Apparently when the Neg'Var model got to customs they thought there might be drugs hidden in it so they broke it open. When it came back it was broken in pieces. That was a nasty surprise." (Cinefantastique, Volume 28, No.4/5, page 72) While being repaired, modifications were performed on the model; the bow fins and the nacelle spikes were removed, the long frontal antenna feature was shortened, and pods on the bottom of the wings were added to the original model. Another modification was the change in color scheme from somewhere between the dark green of the Bird-of-Prey and the light gray of the Enterprise-D (the color scheme for the Vor'cha), back to Klingon dark green.
The Negh'Var was intended to represent the most powerful ship in the Klingon fleet as Gowron's flagship. As the actual studio model was relatively small, 33"×24", the producers decided to have a larger partial ventral side only model built as well, for close-ups purposes. That seven-foot model was nearly three times as long, and though only partial, fully functional with internal lighting. This model also appeared as the Regent's flagship in the mirror universe. In spite that it was referenced in the script as the 'Klingon flagship we saw in "Way of the Warrior"', the design appeared to be several times larger in scale. As with "The Way of the Warrior", both Hutzel and Glenn Neufeld worked on the visual effects for "Shattered Mirror", the first episode the Regent's flagship appeared in. Since the episode had five times the effects budget of a normal show, a huge battle scene was created for the climax of the episode, so it was felt that it would be time effective to bring in both visual effects supervisors.
For the battle sequence between the ISS Defiant and the Klingon flagship, it was necessary to create a massive ship that would completely dwarf the Defiant. The only suitable model the effects team had was the Negh'Var, however, the problem was that the Negh'Var model was only slightly bigger than the Defiant model, two and a half feet long, but to achieve the appropriate shots, the model for the Klingon flagship would need to be roughly twenty-five times larger than the Defiant model, over sixty feet long. Since creating such a model that size was not a viable option, Neufeld got approval from the producers to modify the battle sequence slightly. Originally, the Defiant had flown all around the Klingon ship, but Neufled changed it so that the Defiant only goes in close to the underbelly of the ship. This allowed model maker Tony Meininger, repeating what he had done for "The Way of the Warrior" but on a larger scale, to construct a twenty-five foot model of the underside of the ship only, which was then for extreme close-ups filmed in standard Motion control photography with the Defiant model (Star Trek: Deep Space Nine Companion (p. ?)).
The Negh'Var design eventually became a CGI model, built at Foundation Imaging, that incorporated both the elements of its TNG and DS9 versions to become another future variant only used in "Endgame". Discounting the 2006 remastered Original Series, it was the very last existing physical studio model to be translated into a CGI version for a running Star Trek television production.
The seven-foot ventral side studio model, measuring 84 × 68 × 13 inches, was eventually sold as Lot 4668 in a It's A Wrap! sale and auction for US$7,500. A white foam-core camera test model of the Negh'Var, which measured 24×29.5×7 inches and was used in the production of "All Good Things...", sold for US$885.00 as Lot 7357 in the same auction run.
The original studio model itself was, upon termination of the DS9 series, shortly returned to the tour circuit, appearing in the 2002-2003 Star Trek: The Adventure exhibition tour.  The model was eventually put up for auction and listed in Christie's 40 Years of Star Trek: The Collection auction catalog as Lot 699. The catalog estimate for the lot was US$4,000–$6,000; it ultimately sold on 7 October 2006 for US$22,000 ($26,400 with premium). Sternbach's redesign sketch was earlier offered as Lot 290 in the The Ultimate Sci-Fi Auction of 26 April 2003, estimated at US$400–$600, where it sold for US$400.
The following specifications, which describes this vessel as a Negh'Var-class warship, comes exclusively from the Star Trek: Deep Space Nine Technical Manual:
- Production Base: Qo'noS Orbital Factory Base
- Type: Heavy Carrier
- Accommodation: 2,500 plus flight crew and troops
- Power Plant: Two M/A warp system; four impulse systems
- Dimensions: Length, 682.32 meters; beam, 470.09 meters; height, 136.65 meters
- Mass: 4,310,000 metric tonnes
- Performance: Warp 9.6
- Armament: Twenty ship-mounted disruptor cannons; one large forward disruptor; four torpedo launchers
In the video game Star Trek: The Next Generation - Birth of the Federation, this vessel is designated as a "strike cruiser" and was primarily a torpedo platform used in long-range assaults against starbases and outposts and for planetary assaults. However, in the Star Trek: Armada series of PC games, they are referred to as battleships, and possess more standard armaments.
In Star Trek Online, the Negh'Var-class, along with its sister ship: the Qu'Daj-class, is a battle cruiser available to KDF players of the Brigadier General rank. Negh'Vars also appear as enemy battleships in numerous Federation campaign missions. The Negh'var-class in the game originally lacked the weapon pods seen mounted under the hull in most of the class' televised incarnations, though this was rectified with the "Delta Recruit" event wherein the Negh'Var-class was remodeled to more closely resemble both the TNG and DS9 versions.