Yeah I'm pretty sure it's the thorn one right.? After you do the crucible thing youll have to do the moon strike on highest difficulty and get to the last boss. Don't kill him! Kill all the other enemies and on the last wave of enemies a mini boss will appear. After you kill him, the quest is done. You don't even have to finish the strike. Then go back to the speaker and claim your exotic hand canon: Thorn.