The Destiny 2 Devil's Ruin sidearm was the premier Exotic for the Season of Dawn, and it's still one of the most interesting sidearms in the game. It's pretty potent as a standard sidearm, and by holding the trigger, you can charge and a fire its sub-mounted laser. Charging the laser reloads the magazine, so you always get a full charge. This effectively extends Devil's Ruin's magazine, which makes it great for clearing groups. The best part is that its quest is incredibly short and actually pretty fun, especially compared to standard kill-the-dudes quest grinds. Here's how to get Devil's Ruin in Destiny 2.
The Exotic quest for Devil's Ruin is called A Moment in Time. To start this quest, you'll need to clear the Sundial. It doesn't matter whether you do a normal or Heroic run. Once you clear it, the quest will be available from the rewards pool at the end of the activity. Grab it and talk to Saint-14 in the Tower hangar. He'll tell you to visit Twilight Gap in the EDZ to collect fragments of the weapon.
This leads us to the final step of the quest. Yes, it's that short. Twilight Gap is a returning Crucible map, but you can access it for this special mission via a quest marker in the EDZ. Note that you can do this with a fireteam, and you may as well. Progress is shared, and many hands make for light work.
To complete this mission and claim Devil's Ruin, you'll need to find 10 destroyed battle bots scattered around Twilight Gap and claim the Timelost Fragments they house. We've drawn up a route showing where to find all 10, working roughly counter-clockwise from where you spawn in. That being said, here's where to find all 10 Timelost Fragments.
Voila! Devil's Ruin acquired. Stick around in Twilight Gap to listen to some top-shelf banter, though.