I got one yesterday on the moon during a patrol mission (fetch quest), killed one of those swords of Crota that had spawned in my quest area and it just popped up on my screen! It's nice I was using it since I've heard all the hype about it since it can evolve to an exotic. But then I saw you needed rank 4 Crota and I've been spamming her bounties and I'm at 1 1/2 her rep, so perhaps getting another exotic would come by faster than having to spend a month spamming her bounties