The ship's description from FFG is as follows:

"The Onager-class Star Destroyer is a specialized platform for some of the Empire’s most dangerous weaponry. The arrival of its ominous silhouette in orbit heralds a bombardment by massive particle cannons, or, more rarely, a devastating pulse from superheavy composite beam turbolasers. Its deployment in more sensitive Imperial operations can be neither confirmed nor denied."

That sounds so badass I wish this ship was included in the movies!

Commissioned by Fantasy Flight Games
Art director: Preston Stone