Weapon Perks:
Shock Blast Fires high-damage Arc energy that over penetrates enemies.
Full Bore Increased range at expense of stability ability and handling speed.
Accurized Rounds Increases range.
Long March Radar detects enemies from farther away when weapon is equipped.
Fitted Stock Higher stability, lower handling speed.
After completing the game and you continue to explore the world of Destiny 2, you will get a mysterious item randomly in your Power Slot. The item tasks the player with heading to Nessus and collecting some items from Cabal. Once thats done, you get the next, mysterious step: Await the World-Eater. The step comes with the following text:

The decrypted communications indicate something is coming. Look for the arrival of the World-Eater.

Now that the Leviathan raid is live, the Await the World Eater Step becomes Destroyer of Worlds which simply requires that you complete the raid. After that, its time to take on a very tough version of the Arms Dealer Striker. Its pretty much a Prestige Nightfall with only 7 minutes to start. You must destroy anomalies that appear in order to get more time. When you beat the strike youll actually get the weapon, though it will only have a single perk, youll need to complete some more steps to have the full weapon.

Back to the Bazaar is the next step in the quest to acquire the Legend of Acrius, and it requires to speak to Benedict 99-40 who has a few objectives for you:

Kill 25 Cabal
Kill 15 Cabal at Close Range
Get 10 multi-kills on Cabal with the Legend of Acrius. Since the weapon only carries one slug in the magazine, youll have to work at grouping them up.
When youre all done, return to Benedict 99-40. The next step is His Highnesss Seal. You must eliminate bosses and complete the Leviathan raid to earn 100 Emperor Seals.