In Destiny 2, Osiris picked up a distress signal within the Vex Network sent by Saint-14, and it's up to you to figure out how to help.
Step 1: Open a gateway. Your first task for Osiris is to place transponders. You'll need to travel to Nessus to do this, and you need to visit six different locations. When you get to each location, there is a small white circle you need to stand in to crash the transponder. Hold down the corresponding button and your keeper will stab him into the ground. Here is everyone's card. The only icon not shown on the map was the transponder in the lower left corner, southwest of the luminance pools.
The hardest part is at the top, in the Water Chamber. You have to use the teleporter, right next to the big golden ship to access it.
Stage 2: Saint's Ghost. The transponders identified the ghost of Saint. To find it, you'll need to quickly go to the cistern spawn point and go up, then left. Continue on the path inside of it, avoiding the Vex Milk and the Vex inside. You will be given waypoints inside for things to do, like interacting with a Vex terminal, then you will have to jump through the portal to enter a hidden Vex chamber. The ghost is inside.
Step 3: Take him home. Now that you have the Ghost of Saint-14, you can bring it back to Osiris.
Step 4: Recharge the sundial. You must use the sundial, but it requires recharging. It's your job to give him the juice he needs. You must now 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 a couple of Gambit or Cruicle matches, so be sure to visit Drifter or Shaxx before you jump into this task.
We will update this guide as we complete it.