Onager-class Star Destroyer

"Should the Empire abandon theatrics, it could build a thousand small cannons for the cost of a single "planet-killer.""

- Haxen Delto, in a report to Airen Cracken

The Onager-class Star Destroyer, nicknamed the Siege Breaker by members of the Rebel Alliance, was a type of capital ship and superweapon deployed by the Galactic Empire.

General characteristics
"I'm just not convinced you need a moon-sized reactor to power a kyber crystal cannon."

- Haxen Delto, to Airen Cracken in a report

The Onager-class Star Destroyer, referred to as the Siege Breaker by the Rebel Alliance, was a type of capital ship that was propelled by five engine units in the stern of the starship. A solar ionization reactor was located in the ventral hull, characterized by its dome shape. The bridge was situated on the command tower in the center of the ship towards the rear, and the bridge itself was located on a near-trapezoidal structure.

The front of the ship was defined by two prongs which defined the bow as a T-shaped bow with a long indentation in the center that cut through to almost midship where the primary armament was located. Past the ending of the indentation but before the command tower structure, were two deflector shield generators similar to those found in Imperial-class Star Destroyers. The Onager-class had two hangar bays, one located on the rear of each of the two prongs that form the T-shape of the bow. The Onager-class was able to receive starships, such as a Lambda-class T-4a shuttle and TIE starfighters.

Armament
"Maybe I lack the Empire's taste for catastrophe?"

- Haxen Delto, to Airen Cracken in a report

Located in the indentation was the primary armament of the Onager-class, which were the two superheavy composite beam turbolasers considered to operate as special battery armaments. These turbolasers were speculated to be powered by kyber crystals by members of the Rebel Alliance. The Rebel Alliance believed these kyber crystals were powerful enough to shatter mountains and anything but the strongest energy shield, and as the ship was considered a superweapon by them.

The foundation of the weapons were at the end of the indentation by the bridge, and each weapon had at least three rings that were separated along the length of the indentation through which the payload of the weapons passed through. When these armaments fired, the indentation and rings glowed red, and could fire almost simultaneously. These two superheavy composite beam turbolasers were considered some of the Galactic Empire's most devastating weaponry and could be used to perform ignition attacks beyond long range, allowing it to keep the crew of the vessel safe. In concert with these two primary weapons, the Onager-class was also equipped with massive particle cannons.

Role
The Onager-class Star Destroyer was used by the Galactic Empire as a specialized orbital bombardment platform. The Rebel Alliance believed that the Siege Breaker, if their intelligence was correct, acted in the capacity of a portable cannon. The deployment of Onager-class Star Destroyers in more sensitive Imperial operations had the characteristic of not being able to be confirmed of denied.

Early history
Sometime between the years 5 BBY and 0 BBY during the early rebellion against the Galactic Empire, Alliance Intelligence made reports on possible superweapons the Galactic Empire could be developing. Alliance Colonel Haxen Delto and his group used a lead that led him to believe that the Empire could build thousands of relatively smaller cannons, which the Rebel Alliance dubbed Siege Breakers, for the cost of a single "planet-killer." The document also included possible speculation on the characteristics and armament of the Siege Breaker, and an illustration of a Siege Breaker was created. Delto used the document to pursue the principle that the Siege Breaker was more likely than the a large "planet-killing" battle station. The report that featured the Siege Breaker was addressed to Chief of Intelligence General Airen Cracken and was later included in The Rebel Files, a central data repository that contained sensitive non-electric documents. Eventually, it was discovered that the "planet-killer" was the Death Star, a battle station that had the ability to destroy entire worlds.

Service during the rebellion
The information gathered by Alliance Intelligence was correct, including the illustration. The Siege Breaker was officially designated as the Onager-class Star Destroyer and was deployed by the Imperial Navy to various operations, with the more sensitive operations not being able to be confirmed or denied. There were several examples of the Onager-class, with two being the Rakehell and the Cataclysm. The Rakehell at one point was accompanied by two Victory-class Star Destroyers while the Cataclysm was stationed near a pair of gravity wells and was receiving a Lambda-class T-4a shuttle and two TIE starfighters.

The Onager-class Star Destroyer remained active after the decisive Battle of Endor that resulted in the destruction of the second Death Star and the death of Imperial Emperor Palpatine, and was still considered to be new and technologically advanced. Imperial forces that survived the Battle of Endor made strides to strengthen their grip on the galaxy and launch a counterstrike against the Rebel Alliance. The remains of the Empire believed that the Onager-class Star Destroyer was critical to achieving victory in the final stages of the Galactic Civil War.

Legacy
In 34 ABY, The Rebel Files were uncovered during an excavation on Durkteel and eventually reached the Resistance, a private military with roots in the Rebel Alliance and supported by some members of the New Republic that was locked in a cold war with the First Order, the successor government of the Galactic Empire. Resistance Major Caluan Ematt reviewed the documentation related to the Onager-class Star Destroyer and left a written annotation describing that he believed that the First Order may have used technology or inspiration from the Onager-class Star Destroyer, and would represent a more containable risk. After war erupted between the First Order and the Resistance, the First Order Navy deployed the Mandator IV-class Siege Dreadnought, which functioned and was armed in a similar capacity to the Onager-class.

Behind the scenes
The Onager-class Star Destroyer was first mentioned and pictured in the canon boxed set and reference book Star Wars: The Rebel Files as the Siege Breaker. The book was authored by Daniel Wallace, had illustrations by Joe Corroney and was originally released in November 21, 2017. It was first identified as the Onager-class Star Destroyer in a news article by Fantasy Flight Games on its upcoming Star Wars: Armada expansion packs, Onager-class Star Destroyer Expansion Pack and Nadiri Starhawk Expansion Pack, to be released in 2019.

Lucasfilm Story Group executive Leland Chee confirmed that in developing the Onager-class for Star Wars: Armada, the design came first. When asked how the design for the Onager-class came to be, Chee stated that the Onager-class was designed for The Rebel Files. When Fantasy Flight Games saw the design, it was further developed for their product as the design was appealing and fulfilled a gameplay need.