The Nebula-class Star Destroyer (not to be confused with the class used by the United Federation of Planets of the same name), also known as the Defender-class Star Destroyer, was the largest, most powerful warship design in the Alliance of Civilization's New Class Modernization Program.
Although it was only 1,040 meters long (65% of the length of an Imperial-class Star Destroyer), the Nebula-class was built to be a match for an Imperial II. Even with a structure that was a significant departure from the Imperial-class, especially the lack of a command tower, the Nebula was often mistaken for an Imperial design due to its angular lines. Its small size often lead to it being referred to as a "pocket Star Destroyer", much as the later Turbulent-class would be as well.
It was designed to be able to defeat any one enemy Star Destroyer, two heavy cruisers, or an entire line of smaller Imperial support ships. It mounted defenses strong enough to resist even some of the smaller Super Star Destroyers or analog warships.
The armament of the Nebula-class Star Destroyer consisted of 40 heavy dual turbolaser cannons, 40 turbolaser cannons, 20 ion cannons, 8 tractor beam projectors, and 8 assault concussion missile tubes, each equipped with 30 missiles. Despite the touting of far superior firepower over the Republic-class, the two had nearly similar armaments: the difference was eight concussion missile launchers for the Nebula, and two additional tractor beams for the Republic. The power generation and model of guns was different enough to make the Nebula a far superior design to the Republic. In addition, the hull armor and shield systems of the Nebula outclassed most other ships in the galaxy, including the Republic. 
Originally conceived as part of the Defender program during the first war, the plan for a “pocket Star Destroyer” was later folded into the larger New Class program. While only two-thirds the length of an Imperial Star Destroyer, the Nebula had enough weapons to be a match for an Imperial II and enough armor to resist the firepower of some smaller Super Star Destroyers. It is first seen in service following the events of battle for Yurra Jang, and despite playing a major role in the Second War, the number of completed Nebula’s remained low compared to the other New Class designs and the rest of the fleet. The Cenarius commanded by Malfurion Stormrage, belongs to this class