Black Hammer is only available to lucky players who were able to complete the raid, and from there, they have to be pretty good to get the special effects to kick in and wield it well. On the other hand, unless you want to see level 28 players killing Atheon on hard mode, Xur should probably not sell the Gjallarhorn for now. At least wait until the Wolfpack Rounds lose a little bit of their bite via a slight patch to damage.