For the Impossible Task mission, you will need to track down Saint-14’s Ghost. If you want a chance to rescue Saint-14 from death, bringing back one of the most legendary Guardians the Tower has ever known, you’ll need to find his Ghost first.
To do this, head to Nessus in your Director, and spawn at The Cistern. You want to head straight for the gap in the wall that is in front of you and to the right. Go inside and head all the way through the tunnels. You’ll need to avoid some Vex Milk, but keep going, and head deeper into the Pools of Luminance.
Eventually, you will come to a room with enemies in it, and a stream of Vex Milk flowing on the ground. You can rush past them if you wish, or kill them to try to farm more Dawning Ingredients. When you eventually reach the final chamber, you will see a massive Vex Gate on one side, and a Vex Construct on the other.
Interact with the construct to activate the gate. You can, once again, choose to either farm the enemies in this room or ignore them. When the Vex Gate activates, just run through it and then turn to the right. At the end of the hall, you will find a pile of broken Vex, and one of them is grasping a Ghost in its hand.
Interact with the Ghost to grab it, and then return to Osiris for the next part of the quest.