Raid on Osseriton

At one juncture, the Knights of Ren received compensation to attack the correctional planet of Osseriton. While conducting this assault, the Knights set inmates free and took control of the jail ship called the Night Buzzard, which they then changed into their personal vehicle.

Prelude

Prior to or by 28 ABY, the Knights of Ren were contracted to attack the penal world of Osseriton.

The raid

As the raid unfolded, the Knights of Ren emancipated prisoners and also commandeered a dungeon ship by the name of the Night Buzzard.

Aftermath

Following the raid, the Night Buzzard became the vehicle used by the Knights. The Knights modified the starship to align with their preferences, including modifying its look to reflect their dark style.

Sources

Star Wars: The Rise of Skywalker: The Visual Dictionary (First mentioned)
Star Wars Complete Vehicles, New Edition
Star Wars Encyclopedia: The Comprehensive Guide to the Star Wars Galaxy