You’ll need the Ghost of St. 14 to complete the “impossible task”. If you want to save Saint-14 from death by bringing back one of the Tower’s most legendary Guardians, you must first find his Ghost.
Go to Ness and revive yourself in the Reservoir. Go straight to the crack in the wall in front of you. Go inside and go through the tunnels. Continue downhill until you are at the Pools.
Eventually, you’ll end up in a room with enemies and a stream of Oil of the vex. After dealing with your enemies, go to the last room, where I will be a massive gate.
Interact with the construction to activate the gate. You will be met again by enemies – kill them or pass by. When the gate is activated, walk through it and then turn right. At the end of the hall, you will find a bunch of broken vex, and one of them will have a Ghost in his hand.
Interact with the Ghost to grab him and then go back to the Osiris for the next part of the quest.