In Destiny 2, Osiris has picked up a distress signal from within the Vex Network sent by Saint-14, and it’s up to you to figure how to help.
Step 1: Open a Gateway. Your first task for Osiris is to place transponders. You’ll need to go to Nessus to do so, and there are six different locations you need to visit. When you arrive at each location, there’s a small white circle you need to stand in to plant the transponder. Hold down the respective button, and your Guardian will stab it into the ground. Here’s the map of each one. The only icon not shown on the map was the transponder in the bottom left corner, to the southwest of the Pools of Luminance.
The toughest one is at the top, at the Chamber of Water. You need to use the teleporter, right next to the large, golden ship to access it.
Step 2: Saint’s Ghost. The transponders have pinpointed Saint’s ghost. To find it, you’ll want to fast travel to the spawn point at The Cistern and go to up, and then the left. Continue down the path inside of this, avoiding the Vex Milk and the Vex inside. You’ll receive waypoints inside of what to do, such as interacting with a Vex terminal, and then you need to jump through the portal to enter a hidden Vex chamber. The Ghost is inside.
Step 3: Bring Him Home. Now that you have Saint-14’s ghost, you can bring it back to Osiris.
Step 4: Recharging the Sundial. You need to use the Sundial, but it requires a recharge. It’s your job to give it the juice it needs. You now need to defeat 25 Guardians in Crucible or Gambit matches, get 25 kills using your Super, and acquire 25 Orbs of Light. You can probably do this quickly by running two or three Gambit or Cruicle matches, so make sure to visit Drifter or Shaxx before embarking on this task.
Step 5: Return to the Sundial. After you have enough energy to charge the Sundial, return to it on Mercury and give it the power it needs.
Step 6: Back to the Past. You now need to succeed where Osiris failed, which was saving Saint-14. Approach the Sundial to the right of Osiris after receiving this step, and go into the past. You’ll start Corridors in Time 2. You’ll go through a standard quest, so you won’t have to do much farming for this next portion.
Step 7: Journey’s End. After this step in the quest, you’ll find yourself in Orbit. You need to return to Osiris on Mercury and speak to him about having saved Saint-14.