Harbinger was a Tector-class Star Destroyer in service to the Imperial Navy during the Galactic Civil War. In 4 ABY, the Star Destroyer was part of the Imperial fleet that defended the DS-2 Death Star II Mobile Battle Station over the Forest Moon of Endor.
After Emperor Palpatine lured the Rebel Alliance into assaulting the seemingly incomplete battle station, the Imperial fleet trapped them over the moon. However, after the rebels engaged the Imperial forces, the Harbinger was destroyed by advancing Alliance vessels, followed shortly after by the Death Star itself and a large portion of the Imperial fleet.
Behind the scenes
The Harbinger was mentioned in the short story "The Levers of Power," which was written by Jason Fry and included in the 2015 compilation book The Rise of the Empire. On Twitter, Fry stated that he intended the Tector-class Star Destroyer seen in the 1983 original trilogy film, Star Wars: Episode VI Return of the Jedi, to be the Harbinger. However, this was not specified in the story itself.