It's been reported on the moon, but I've personally picked up the sword a half dozen times already on earth. It looks like the hive are infecting the zones, so look for them in typical enemy spawn caves. In particular: Inside the first cave to your right on earth from starting point and in both dreg spawn locations at walker public event to the right. Given that this is a hive dlc, this is probably going to happen in all enemy spawn zones. TBD. Either way they heard us when we said that Crota was the favorite mission, now anyone can have some fun at random. :)