Void because most enemies are weak to it XD I'll be getting my exotic tomorrow too.
English
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I believe when you have all the perks unlocked on your sword and have it 280 or higher attack value the quest will appear at Lord Shaxx. That's all I had to do, not sure if I had another prerequisite completed that I didn't know about though
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At 280 you unlock a 5th perk. Whenever it's up graded it will appear to shax as a normal quest. After you finish that you will get the exotic quest (which you get the god awful first step, especially for void on warlocks.
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Dude I picked the void sword on my hunter and it was painful. Getting 500 kills with void abilities sucked but I ended up getting in a groove on the dark beyond mission. It took me around 45 minutes or so, farming for the relic crystals was just as bad hahaha