Forbidden Legacy is a Side Quest in Horizon Forbidden West (HZFW). This walkthrough will guide you through all objectives of the Forbidden Legacy side mission.
Location: The Base / Legacy’s Landfall
Quest Giver: Alva
Quest Level: Level 30
Requirement: After talking to Beta at The Base during Main Quest 14: Gemini.
Reward: 7500xp, 3 Skill Points
Mission Info: Alva believes data in San Francisco may help the Quen fix a defunct flood control system back home. She is determined to find it – no matter what obstacles stand in her way.
Objectives:
- Go to Legacy’s Landfall
- Follow Alva to the Research Facility
- Kill the Slitherfang
- Search the Ruins for the Data
- Follow Alva to the Atbay Headquarters
- Find an Entrance to the Atbay Headquarters
- Unlock the Door
- Search the Headquarters for the Data on Leviathan
- Climb up to the Ancestor’s Office
- Unlock the Door
- Search the Office for the Data on Leviathan
Starting Location: The Base / Legacy’s Landfall
This quest has two potential starting locations. During the story, during Main Quest “Gemini” after talking to Beta at The Base, the quest will become available from Alva. If you go for the quest after the story, it will start at Legacy’s Landfall.
Starting Location #1 – at “The Base” during the story, unlocks after talking to Beta at The Base during Main Quest 14: Gemini.
Starting Location #2 – at “Legacy’s Landfall” after the story:
Go to Legacy’s Landfall
Travel to Legacy”s Landfall and call Alva at the campfire.
Follow Alva to the Research Facility
Now follow Alva to the Research Facility.
Kill the Slitherfang
Once you arrive at the ruins, there will be some dialogue. A Slitherfang will then appear, kill it.
Search the Ruins for the Data
Go up to this vent and press 
to kick it down and enter the building. Swim forward, then get out of the water on the right.
Go forward and turn around to see an obvious path to the upper floors, climb up and go right.
Work your around to the right and the other side of the upper floor and interact with the console here.
Follow Alva to the Atbay Headquarters
Now follow Alva to the Atbay Headquarters.
Find an Entrance to the Atbay Headqaurters
Once you have followed Alva all the way, pull out the wall ahead. Go forward and climb up above here and to the left, upwards and right again.
Climb up a few more grab points and then left again, all the way around and climb up.
Pull down the beam ahead and go across it, shoot the ladder on the right to lower it, then climb up.
At the top are a couple of glinthawks, kill or sneak past them, behind them is wall you need to climb directly up.
At the top, look to the right to see another wall to climb.
At the top of this wall and just ahead is some debris, with two grapple points you need to pull down, right next to each other, once clear jump into the building ahead.
Unlock the Door
In this room is a door you need to unlock. The code is 102023 (Alva basically gives you this if you can the scan the datapoint to the left of the door, it’s the date the Atbay company was founded).
Search the Headquarters for the Data on Leviathan
Go through the door and scan the datapoint near the door, it will play automatically. Then open the door on the left and go through.
Climb up to the Ancestor’s Office
Just ahead on the elevator shaft on the right is some rubble you need to pull out.
Go through the open door back outside, look to the right and jump to the grapple point, then jump on the pipe to its left, climb up and down it a couple of times to make it fall to the left, then walk across it to find some ledges.
Jump across the ledges here, then further up the side of the building, around the right, then up to a ledge.
From here pull out the vent directly ahead, revealing a grapple point.
Grapple to this, then climb down, wall jump to the opposite side, climb a little further up to another pip sticking out of the wall, wall jump to it.
Look up and pull out a beam, jump to it, then up to another ledge on the left.
Turn around and look up to see a grapple point, jump and then grapple up to it.
Turn around to grapple to another point, then look up and grapple to another one.
Look up to the right, grapple here, then a final grapple point just ahead and go into the building.
Go through the doorway and around a corner to see some Firegleam, ignite it.
Once you have destroyed the wall, you can now climb straight up the shaft to another floor.
Unlock the Door
There is another locked door here, the code is 402625 (Alva gives you the numbers if you scan all 4 artifacts in here, the final 2 artifact numbers are in the opposite order, 625 not 256).
Search the Office for Data on Leviathan
Go through the unlocked door leading outside and there is a console you can interact with, after a cutscene the quest will be complete.
This finishes Forbidden Legacy Side Quest in Horizon 2: Forbidden West.
For all other Side Quests and Main Quests, check out the complete Horizon Forbidden West Walkthrough.
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