From VG247: “Its time to save a legend in Destiny 2, so have a word with Osiris and get ready for a fight.
Now that youve spoken briefly with Saint-14 and retrieved his Perfect Paradox shotgun, youll now be able to go on a mission to rescue him from the Infinite Forest and succeed where Osiris failed. The Power requirement for this quest is around the 900 Power mark, so you might need to grind it out a bit before you get started.”
Source: N4G PC Destiny 2: How to return to the past and save Saint-14