Talk to Lord Shaxx and get the quest. It's a long quest chain.
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I've probably got it already, but is there a way to initiate it?
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The Quest it starts with is a Quest that requires you to get 8 kills while your team holds 2 or more Control Zones. That's only the beginning, though. After a while, he'll just start handing you a ton of Quests that take a [i]really[/i] long time to finish. When you finally finish it, you get a 150 Legendary Rocket Launcher. It's really a great Questline.