To acquire the Forerunner Exotic Sidearm in Destiny 2, you must complete the Magnum Opus quest, which you can begin by interacting with Xur. Start by ensuring you have access to the Bungie 30th Anniversary Pack and complete the prerequisite quest To the Daring Go the Spoils, which involves a round in Dares of Eternity and a visit to Xur. Collect seven Strange Coins through various activities, then purchase and complete three bounties from Starhorse within Xur’s Treasure Hoard. You need to enhance your Strange Favor rank to Brave by participating in more bounties and activities associated with Dares of Eternity.
Once you have achieved the necessary rank and obtained the Strange Key, use it to unlock a special area where you will retrieve an important item for your quest. Conclude the quest line by returning to the Tower and speaking with Banshee-44, which will grant you the Forerunner Exotic Sidearm. This sidearm, inspired by the iconic Halo magnum, combines the precision of hand cannons with the rapid fire and reload speed of sidearms, making it a unique addition to your arsenal. For a detailed guide on each step, resources such as Shacknews and Highground Gaming provide comprehensive walkthroughs.