Where to use Splicer Relay Key and find the Khvostov schematic
This video will lead you to an area that requires a splicer relay key. You can get these from wizards called “Brood Mother” near the patrol point of the plaguelands.
The schematics are where everything started in destiny. But you need to begin your trip from the plaguelands patrol point, so it is quite a trip. Defeat all enemies in the airship at the end and you will be able to open the chest with the Khvostov schematics.