First, you need the Khvostov 7G-OX schematic to unlock the questline. There are two ways to do this:
- Dismantle the original Khvostov weapon
- Use a Splicer Intel Relay to board the Ketch ship in The Plaguelands
If you managed to keep hold of that weapon from way back in 2014, then great - pull it out of your Vault, dismantle it and move on to the next step. Of course, most people got rid of it as soon as possible, and thankfully there's another way to get the Khvostov 7G-OX schematic.
You need to head to the place you started Destiny, just outside the Cosmodrome walls. Fire up The Plaguelands patrol (complete the game's final story mission,
The Iron Tomb, to access this) and head back in the direction of the Rocketlands (from where you spawn in, go right, through the doorway on the upper walkway). Once in the Rocketlands, go left to The Divide, then left round the corner into the sewers to access The Breach.
You need to make sure you have a
Splicer Intel Relay on you, because before you can go exit the wall, you'll have to open a doorway in The Breach. (Note that this won't work if you try and access the ship from story The Walls Come Down - we tried it.)
Once you do, work your way through the pile up of cars to the Ketch ship in the distance, and kill everything on board, including a rather persistent Pilot Servitor. Once taken care of, a chest will appear in the far right corner, containing the Khvostov 7G-OX schematic.
Whichever way you get it, take it to the Iron Temple to continue the next part of the quest.