"She's a predator."
The Reman warbird Scimitar was a massive, heavily-armed warship. It was constructed under the command of Shinzon as part of a secret plot to overthrow the Romulan Senate, defeat the United Federation of Planets, and liberate the subjugated natives of Remus.
Built at a secret base, the Scimitar was the keystone of Shinzon's plot to assassinate the Romulan Senate and assume the role of Praetor of the Romulan Star Empire, something he could not have achieved without the support of the Romulan military. The Scimitar helped prove Shinzon's ambitions of attacking the Federation and ending the period of peace that had frustrated some Romulan generals.
The Scimitar inflicted severe damage on the USS Enterprise-E and Romulan reinforcements during the Battle in the Bassen Rift. When the crew of the Starfleet ship made a successful attempt to collide their vessel with the Scimitar, the Reman warbird was crippled. As Shinzon made a last ditch effort to inflict even more damage, a powerfully destructive thalaron generator aboard the ship was powered up, though Captain Picard boarded the vessel in a failed attempt to disable the weapon. Lt. Commander Data of the Enterprise managed to board the Scimitar, where he transported Picard off the ship. Data fired his hand phaser into the thalaron intermix chamber, only two seconds before the weapon would have fired. This caused a chain reaction which destroyed the ship, and Data along with it. (Star Trek Nemesis)
The Scimitar's cloaking technology was, in the words of Geordi La Forge, "perfect." It emitted no tachyons and left no residual anti-protons, the two ways that a cloaked ship could be tracked and located. Thus, when cloaked, the Scimitar was virtually impossible to find. Only a chance hit by an opposing ship's weapons or by targeting where the Scimitar's shots came from could reveal its location, temporarily disrupting the cloak. The Scimitar could even deploy its weapons and deflector shields and travel at high warp speeds while cloaked, actions which, with less-advanced cloaking technology, necessitated de-cloaking or reduced the effectiveness of cloaking. Its maximum warp factor when cloaked was more than that of Starfleet's Sovereign-class starship.
Weaponry and military equipment
Having been designed for war, the Scimitar had 52 disruptor banks and 27 photon torpedo bays; it was thus ready for nearly any battle, carrying roughly five times the armament of any other single ship. The powerful thalaron radiation weapon with which it was also equipped was capable of stripping the life off a planet in a matter of seconds, though it took seven minutes to prepare. The Scimitar also held several Scorpion-class attack fighters.
The Scimitar had primary and secondary shields. This allowed for twice the protection, giving it a backup shield. Even against the USS Enterprise-E and two Romulan Valdore-type Warbirds, the Scimitar still had the obvious advantage.
The bridge of the Scimitar was radically different in design from any other vessel, though it did share the same basic layout as most Romulan starships.
In the center of the room was located the chair of the vessel's commanding officer. Behind his chair, both to the left and right, were consoles that required the operator to stand up. Directly in front of the captain's chair were two more stations. The command chair itself contained a built-in replicator, for the commanding officer's needs.
Directly behind the captain's chair was a flight of stairs that led to a higher level. Here, two more consoles were located, and access was provided to the thalaron weapon room through a pair of doors. (Star Trek Nemesis)
The shuttlebay of the Scimitar was a large area over two decks. It featured several Scorpion class attack fighters. The shuttlebay was completely destroyed when the saucer section of the Enterprise-E was crashed into the Scimitar. (Star Trek Nemesis)
After an initial description of the Scimitar as "magnificent," the shooting script for Star Trek Nemesis introduces the craft with the following description; "Our first sight of this incredible ship is absolutely breathtaking. Shinzon's vessel combines the clean lines of the traditional Romulan Warbird with unique weaponry and styling. It is huge, easily twice as large as the Enterprise. And it is aggressive. Awesome in its power." 
The name Scimitar itself is actually derived from an ancient Middle Eastern curved sword, used as far back as the mid-16th century. (citation needed • edit) The name was originally to have been used for the USS Defiant in TOS: "The Tholian Web". (The Star Trek Compendium)
In Star Trek: Countdown, the Remans were shown to have constructed smaller ships based on the design of the Scimitar. Three of these ships were used to capture the Narada, but were easily disabled by the Enterprise-E.
Scimitar-class ships appear in Star Trek Online, classified as dreadnoughts and used by the Romulans as well as the Remans.