Osiris has committed himself to a quest for vengeance after the Hive killed his ghost Sagira. He has called for your aid in tracking down Sagira's killer, the High Celebrant of Xivu Arath.
"Losing a Ghost is a traumatic experience for a Guardian. Total severance from the Light, from the Traveler, from their destiny as a Guardian. I can't even imagine how that feels." —Ghost