To complete the "Impossible Mission" you will need the Ghost of Sant-14. If you want to save Saint-14 from death by bringing back one of the most legendary Tower Guardians, you first need to find his Ghost.
Travel to Nessus and respawn 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 reach the Pools.
Eventually, you will find yourself in a room with enemies and a stream of Vex Oil. Having dealt with the opponents, go to the last room, where there will be a massive gate.
Interact with the structure to activate the gate. Enemies will meet you again - kill them or pass by. When the gate activates, go through it and then turn right. At the end of the hall, you will find a bunch of broken Vexes, and one of them will have a Ghost in his hand.
Interact with the Ghost to grab him and then return to Osiris for the next part of the quest.
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