This is the exotic trace rifle that many associate with the Garden of Salvation raid. The association is most likely due to the face that the final step of the quest requires completion of this raid with a series of puzzles.
This quest line consists of five steps:
Travel to the Lunar Battleground on the moon. Venture into the cave on the left side which is also the start point of Garden of Salvation. Kill a plethora of Vex enemies that will spawn in the cave followed by a large Vex boss. This will give you the start of the quest.
Scan Oracles within three locations on Nessus. Each set of oracles will be found in various lost sectors around the planet.
Kill Vex enemies on the moon to obtain decryption cores.
Buy an Empowered Decryption Core by spending 30 Phantasmal Fragments on the moon.
Complete the Garden of Salvation raid with each puzzle being completed along the way to the end. At the end of the raid, make sure to open both chests that spawn at the end.