How did you get it??

Here lately I haven't played much because I'm starting to feel the weight on my shoulders, I don't find it as fun anymore. A friend of a buddy of mine needed that Eris bounty, to kill 10 ultras in a nightfall, so we ended up running the whole thing because I needed it. Right as we killed the final boss, the friend was like, "No damned way, how do you get the ghallahorn?" But for me I got no message saying I received the weapon, so I played it off as a joke. Then I saw the rewards page and almost fell out of my chair