The Relentless was a well-known Imperial I-class Star Destroyer, most notably serving as a component of Grand Admiral Thrawn's naval forces during his military actions against the New Republic.
Prior to 0 ABY, the Relentless was under the command of Captain Parlan, who oversaw the Imperial forces searching for Adar Tallon located on Tatooine. After his failure in this assignment, Darth Vader executed Parlan. Shortly thereafter, Captain Westen assumed command of the Relentless, but he suffered the same fate when he also failed Lord Vader.
By 4 ABY, Captain Dorja held command of the Relentless. A commander known for his caution, Dorja led the ship during the Battle of Endor, where the Relentless stood out for not incurring any casualties. Subsequently, the Relentless withdrew alongside the Chimaera and remained a part of the fleet under Thrawn's eventual leadership. Due to Dorja's personal conflicts with Gilad Pellaeon, the captain of the Chimaera, and Grand Admiral Thrawn, he was assigned a secondary role in Thrawn's campaign in 9 ABY. The Relentless had a role in the Raid on New Cov, where its crew tried to capture both Han Solo and Luke Skywalker. The Relentless also participated in the seizure of the Ukio system and the Battle of Bilbringi.
The Relentless, still commanded by Dorja, later joined the Imperial Remnant's Braxant Sector Fleet, answering to Moff Vilim Disra and Pellaeon, who had become a Fleet Admiral. In 19 ABY, Disra dispatched the Relentless to intercept a Corellian corvette in the vicinity of Morishim. This corvette was carrying Colonel Meizh Vermel, a messenger sent by Pellaeon to initiate peace talks with the New Republic. Later that year, the Relentless was present at the Battle of Yaga Minor, carrying the Remnant's ruling triumvirate: Moff Disra, Major Grodin Tierce, and Flim, the Thrawn impersonator.
In 28 ABY, after the Imperial Starfleet's retreat from the Battle of Bastion and the withdrawal at Yaga Minor, the Relentless took the lead of Fleet Group Relentless. This fleet group was one of the four temporary battlegroups established for the defense of Borosk against the invading Yuuzhan Vong. Following the battle, the Relentless was among the Star Destroyers that were ordered to be deployed from Borosk to aid in the defense of Yaga Minor. After Yaga Minor, the Relentless went with the Right to Rule on its mission to hunt down the remaining forces of B'shith Vorrik to Generis and then Esfandia, where it was involved in the Battle of Esfandia.