I've killed dozens of the Blades of Crota, and not one of them has dropped this weapon for me yet. I was thinking, since it is a Common and you have to complete the Raid on Hard to get it to Exotic, that maybe Xur should sell it? For maybe 7 Strange Coins? Just a thought...
Alternatively, they could have Eris sell Husk of the Pit once you get to Rank 4 of Crota's Bane, for the same price. What do you think?
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Dude its not that big of a deal. Seriously just do patrol on earth or the moon, the chances are higher than doing the fist mission.