Now you may be wondering why exactly the pinnacle of the Romulan Navy is making an appearance here, in a document about the Federation's battleships. There's certain ways to obtain ships besides construction at least. That aside, what is clear is that the D'Deridex is potentially one of the finest ships of the 24th century.
The D'Deridex owes a lot of its traits, strangely, to the Borg Collective. The Borg and the Romulans can be said to have had an… interesting relationship since their first contact in 2313, shortly after the Tomed incident. At that point in time the Romulan's premier ship of the line was the Keras class, a ship bearing noticeably Klingon lines and reactors based on Klingon systems. The Borg, from what the Federation can gather, appear to have adopted a strategy of "Farming" the Romulan Empire, appearing every few years to assimilate a few ships, causing the Romulans to advance more for the Borg to assimilate more advanced tech rather than putting in the effort to R&D improvements themselves. The Romulans were well aware of this and willingly accepted it, as sending a few more ships out to die every 15 years or so is generally considered worth not being assimilated by the Borg. This line of thinking is what birthed first the Horo class.
The Horo class could be called many things, but backwards it was not. Capable of fighting an Ambassador to a standstill the most infamous event the ships were involved in was the battle at Narendra III, where 4 Romulan ships destroyed a Klingon Colony and two Klingon vessels. The USS Enterprise, NCC-1701-C, moved to intercept but was lost in battle, and is believed to have been captured as a war prize. She's allegedly still on display over Romulus to this day. It was this substantial loss that caused the Federation to put project Galaxy into Overdrive whilst rushing the Niagara class into service. And yet the Romulans didn't stop advancing.
Finally, in 2364, during a minor incident involving a cryogenics satellite from the 1990's and the USS Enterprise, the D'Deridex and Romulus itself appeared on the galactic stage. Not only dwarfing the Galaxy class but appearing in numbers wherever they went the Romulans certainly had a tendency for theatrics with the ships, often using the cloaking devices for dramatic reveals to intimidate Federation and Klingon diplomats alike. One would be forgiven for thinking the Romulans just enjoyed fighting enemies they could actually intimidate. Given there's reports of Federation captains using Excelsiors the same way prior to the banning of cloaking devices in 2312, those who think that are likely correct.
The D'Deridex itself most certainly has the bite to match its bark. Powered by a Romulan Singularity core, feeding its plasma through a Dilithium Intermix Chamber and then either to the ship itself or the warp engines, the D'Deridex is one of the largest ships ever encountered at 1,048 metres long. Capable of making a maximum of Warp 9.2 for 12 hours and Warp 7 indefinitely the ships sport a primary "head" section connected by two huge Dorsal and Ventral wings to both the nacelles and a secondary fin-shaped hull in the stern. Dotted across the ship are no less than 18 disruptor arrays, as well as at least 4 torpedo launchers.
A number of ships were also outfitted to shunt the entire output of the warp reactors out the main deflector, something previously only seen on the USS Enterprise during the Borg Incursion of 2367. Given it's long been suspected the Romulans were trailing that specific Borg Cube, including the finding of Romulan food rations and medical kits aboard a number of the wrecks from the battle of Wolf 359, this was most likely knowledge gained from that incident. Given that the Federation had a substantially larger problem than the Romulans saving Federation lives at the time, this incident has largely been agreed to be a special case.
The D'Deridex, like many ships. got its true showing during the Dominion War. The Romulan entry into the War was dependent on the D'Deridex class, and saw at least 15 Dominion sites struck within 12 hours. D'Deridex's continued to fight alongside Federation and Klingon ships until the final battle of Cardassia, often backing up the very Galaxy and Negh'Var class ships they were meant to counter. Countless Federation Lives can be owed to the sacrifices of Romulans and Romulan ships, and the war arguably redefined Federation-Romulan relations. Even to this point the Federation is making efforts to aid Romulus in recovery and… the future.
As readers will know, in approximately two years the Hobus star will go supernova, destroying Romulus and Remus. Whilst the Federation has shuttered its largest effort to evacuate Romulus due to the Synth attack on Mars destroying the evacuation fleet, official policy is still allowing and encouraging Federation vessels to assist in evacuating Romulus between assignments. The Olympic Class, a transport and Medical Vessel, will be instrumental in this, and a fleet of 5 such vessels has slowly been relocating Romulan leadership to New Romulus. Notably, however, a number of Romulan Captains have broken rank, taken their ships, among them several D'Deridex's, to pick up their crew's families and head for Federation space. The Federation has officially offered to take over operation for these ships alongside a guarantee of safety for any Romulan who accepts or denies the offer.
So far 5 such ships have been officially obtained by the UFP, and used to assist the Olympic class in Romulan evacuations. Whilst some may consider it highly unusual to see a D'Deridex in the glistening light and dark grey of the Federation, the ships have certainly made a name for themselves as transports. A number of other D'Deridex's, declawed and used as freighters by other Romulan defectors, are registered in Federation space. Overall whilst Romulus' time may be coming to an end, their legacy will live on.