This will unlock once you complete the first half of City of the Sun. Erend wants you to investigate his sister’s death with your Focus. Meet him at the Red Ridge Pass. When you get there he’ll be under attack from a group of Machines, so get in there and kill them all to help him out.
When they’re down talk to Erend, then when back in control, follow him to the site of the battle. When you arrive, use your Focus to scan each of the points of interest. These are, in no particular order:
Investigate each of these to initiate a short cutscene where you’ll find cart tracks. When you’re back in control, follow the tracks up the side of the hill to the nearby Ruins. There will be a bunch of hostile Oseram there, so be ready for a fight. There’s a mixture of bow snipers on the walls above you and melee attackers who will try to rush you. Deal with them in whatever way you prefer. When they’re down, a beacon will activate that will call in two Ravagers to attack you. Use Tearblast arrows to blow off their cannons and then pick them up to make short of the Ravagers.
When they’re down, another cutscene will trigger then you’ve got some more detective work to do. Scan with your Focus to find the Leather, Rocks, Bloodstains and the Vanguard Weapons. Once you’ve investigated all of these, use your Focus to find the nearby Tripod and then go investigate it then return to Erend. When the cutscene is over, that’s this quest complete.