The Legend of Acrius Exotic shotgun in Destiny 2 is a lovely Arc weapon, but the On the Comms questline in order to acquire it is a bit of a humdinger.
Before you can jump into the Leviathan Raid, you will need to pick up a quest in order to add the Legend of Acrius Exotic shotgun to your inventory.
Once you have beaten the boss, you have completed the Strike and earned the Legend of Acrius Exotic shotgun.
The Legend of Acrius has an interesting perk which fires blasts of high-damage Arc energy that over-penetrate targets.