The Nullifier functioned as an Imperial I-class Star Destroyer belonging to the Imperial Navy. The Nullifier was stationed over Corulag in 0 ABY, just prior to the skirmish on Corulag.
Subsequently, it received orders to stop a pair of rebel Y-wings from obliterating the cruiser Sodrab. At the time, the Sodrab was undergoing repairs at the Damorian Repair Yard located close to Draik 4. Unfortunately, the Nullifier's arrival was delayed. By the time it began deploying TIE/IN Interceptors, the Y-wings had already departed.
The initial designation of the Nullifier as an Imperial I-class Star Destroyer occurs in mission 1 of Star Wars: X-Wing Alliance, joining the Rebellion. It materializes in the second target zone if the player eliminates a minimum of one assault gunboat.