The Rebel Alliance Nebulon-B frigate Rehz'nor, command ship for Green Squadron, was taking on supplies, presumably in support of Operation: Quick Strike. Having completed the transfer, the convoy ships were heading back to a hidden Rebel base for more provisions when the Imperial Strike-class medium cruiser Scylla dropped out of hyperspace and launched its first wave of fighters.
Green Squadron's two X-Wings departed their hanger to join two X-Wings from Rogue Squadron, already patrolling the immediate area. Both groups engaged the Scylla and her launched TIE/IN Interceptors and TIE Bombers as the Rebel convoy ships began to make their way to hyperspace. The Scylla attempted to maneuver in front of the convoy ships to engage them safely away from the Rehz'nor's guns, but was harassed by Green Squadron's X-Wings. The YT-1300 light freighter Outlaw completed its docking operation and joined the fight briefly, withdrawing to hyperspace after exhausting its missile supply on the TIE craft.
Several minutes into the conflict, as Scylla's fighters were all but destroyed, the Victory-class Star Destroyer Triumph dropped out of hyperspace, launching her own TIE Bomber flight group. Shortly thereafter, Scylla withdrew from the battlefield into hyperspace.
As Rogue and Green destroyed the fighter threat, the Rehz'nor began exchanging fire with the Triumph directly. Aided by Green and Rogue Squadrons' X-Wings, the Triumph's shields were taken down, and she maneuvered to jump to hyperspace, leaving the Rehz'nor damaged but still functional and able to continue operations.
As a two player training mission in the game Star Wars: X-Wing vs. TIE Fighter, the exact time and location of the battle are unknown. It is the only known sighting of the Nebulon-B frigate Rehz'nor outside of the Battle of Gaerboud IV. Presumably this battle took place during that same time period and in the same localized area, as both were included with the initial release of Star Wars: X-Wing vs. TIE Fighter and are the only known sightings of the Rehz'nor.
It is possible to destroy both the Scylla and Triumph during the mission, however the Scylla will exit to hyperspace if left alone, and the Triumph will attempt to withdraw to hyperspace once its shields are down, leaving their exact fate unknown. Likewise, the mission objectives can be met even if one of Cargo Ferry group Gefion is destroyed, making it possible that (the first attacked by the Scylla) was destroyed during the skirmish.
The Triumph in this mission may have been a reference to the Victory-class Triumph released in 1996, the year before Star Wars: X-Wing vs. TIE Fighter in 1997. If so, it would cement the Legends position of the ship being a Victory-class Star Destroyer.