The Flagship of Starfleet's Naval Intelligence Bureau, the Nimrod Class holds the unique distinction of being the Federation's only outright stealth cruiser. Exceedingly large and capable of extreme long range deployments the ships held a fairly respectable career, albeit one still largely under wraps even a century later. Nonetheless it was still a remarkable, albeit remarkably strange vessel.
The Nimrod Class was penned in 2246 due to a niche for stealth vessels being noted in the fleet. The Federation lacked any actual cloaking devices at the time nor any doctrinal concerns related to Stealth, and so it had been low in the priority list. However Starfleet intelligence had requested a vessel capable of covertly inspecting potential Romulan, Klingon and Tholian threats. This vessel would've required extreme long range capability beyond the Constitution, Walker and Shepard Class Ships in service at the time, necessitating a larger ship with greater space for supplies and spare components, as well as a smaller crew. The result was something to be admired.
The Nimrod Class sat at an astounding 570 meters long, the biggest ships in the fleet until the launch of the USS Yamato. Had the ship been public knowledge at the time the USS Nimrod would've been celebrated as the greatest marvel of Federation engineering since the USS Bonaventure left on her first deep space expedition in 2164. Truthfully she was, but due to the ship's nature the vessels were kept so deeply classified that the only way anyone outside of Starfleet Intelligence knew they existed prior to 2318 was by chance glances as the ships passed. The ships weren't placed on the standard NCC roster but classified as NI vessels, further hiding their existence. For a vessel wider than the Sea of the Raging Queen in some places the ships were extremely well hidden.
Backing up the effort into hiding that the ships even existed was their engineering work. The heart of the ship was the SSWR-XV warp core, simultaneously making its shipborn debut on the refits of the Asia class science vessels. This core, the first vertical core ever designed by Starfleet for use aboard a starship, promised far greater power outputs than horizontal cores then in use. Truth be told it was a rather fickle thing, but paved the way for modern vertical warp cores, powering almost all modern ships to this day. This powered a respectable set of shielding emitters and an array of 6 dual FH6 Phaser banks, the best phases available at the time. 2 Fore and 2 Aft torpedo launchers could also be located on the vessel. Interestingly the ships sported a large amount of automated drones kept hidden beneath hull plating. These were meant to be used for covert observation but could also be used for defending the ship in combat. In terms of warp speed the vessel wasn't a slouch either, using 2 pairs of Xian-38 Nacelles to cruise at Warp 7 and achieve Warp 8.5 if necessary, though the ship normally cruised at Warp 5 to produce a smaller warp signature.
The ship's stealth suite is so advanced that it requires its own paragraph. The ships sported a huge array of passive and active systems meant to conceal the vessel when on patrol. The largest and most important was the exterior of the ship itself, coated in a material designed specifically to absorb any and all sensor emissions, rather than bouncing them back. The ships sported an incredible ECM suite as a backup capable of disguising the ship's presence as anything from the USS Enterprise to a passing civilian merchant, though it was equally capable of causing damage to enemy vessels. On top of this was the ship's very warp drive, capable of disguising the minute warp traits specific to Federation starships. Ultimately the only way to tell it was a Federation ship was to look at it directly, something the ship made additionally difficult due to its stark black colour scheme, acting as a final layer of camouflage. In 2272 the ships were refitted with cloaking devices, being among the first ships to carry them.
The ship's careers are largely still classified, but what is known doesn't paint a sorry picture. The first ship entered service in 2253 as the USS Nimrod, and was followed by at least 11 other vessels by 2258. In an effort to reduce costs a second batch of vessels, known as the Hou Yi subclass, was ordered in 2256, with their primary difference being the loss of the lower warp nacelles. Approximately 5 were lost when Starfleet Intelligence's Control AI went rogue, with 3 being visible from the USS Enterprise's security cameras over Xahea when the USS Discovery was lost. The vessels were largely used chiefly in an observational role, keeping tabs on enemy fleet movements. During both Burnham's War and the war of 2268 Nimrod class ships were used to insert spies into the Klingon Empire and subtly cause concerns, namely in Klingon supply convoys, transports and lone destroyers randomly going missing. Consequently the ships gained the nickname "jat'yln" among the Klingons, translating into English as "ghost". During the Genesis incident the ships focused directly on the Klingon Empire, inserting spies to potentially cause more concerns if necessary. In 2294 the ships were quietly retired, recalled home and scrapped, having served their role effectively.
In retrospect the Nimrod was a fairly successful vessel. Though they weren't used in a truly fair fight like Starfleet's more visible tactical vessels, their truest success was never going into a fair fight in the first place. There's believed to be no surviving examples of the ships, though their quiet effort didn't go without appreciation. Ultimately though they were fairly decent vessels, at least as far as what's publicly available goes.
All information in this document is publicly available through Federation archives. No classified material was used as a source for this document.