The Forbidden Legacy is a Side Quest in Horizon Forbidden West, meaning that attempting it is optional. It’s unlocked fairly late in the game i.e. the endgame, specifically in Chapter 3.
In the Forbidden Legacy quest, all you have to do is get your hands on Eileen Sasaki’s data about the Leviathan, which Alva believes would help Quen get their flood control system working.
The Leviathan is a flood control system that the Ancestors built. The Quen have tried to replicate the system but failed due to a lack of necessary information.
Though the main objective is quite simple, the quest is quite a large one and takes a lot of time to complete as you have to fight a powerful giant machine and complete a huge series of parkour courses.
How To Unlock The Forbidden Legacy Side Quest
This side quest is given to Aloy by Alva at the Base. It’s one of the endgame quests that are only available way off in Chapter 3.
Starting this quest is as simple as talking to Alva about it at the Base. Simply select the dialogue option to ask about helping her to start the quest.
Alva needs some data about an old flood control system built by the Ancestors called the Leviathan. She mentions that the Quen tried to replicate that system but never got it to work, so if they managed to get their hands on Eileen Sasaki’s data about it, they could get it to work.
How To Complete The Forbidden Legacy Side Quest
After talking to Alva at the Base and offering to help her (Starting the Quest in the process), she will ask to meet you at the Legacy’s Landfall way over at the western end of the map to get some information from a Diviner, Nirik.
The first step is to head over to Legacy’s Landfall. This shouldn’t be a difficult task as all you have to do is fast-travel to a campfire there.
Once you reach here, you need to call Alva, you can do that by using the option at the Campfire there. Upon arriving, she’ll take you to see Nirik, the Diviner.
After dealing with Nirik and getting the information that she needs, she’ll ask you to follow her to the Research Facility.
Follow Alva to the Research Facility
Upon reaching the Research Facility, Aloy will be met with a large machine – a Slitherfang. This is quite a dangerous opponent so you need to be careful while fighting it.
The Slitherfang Is a giant snake that is susceptible to Frost, Plasma, and Fire damage. You can take advantage of its large size and slow movement speed to damage it. It will often wrap itself around a tower nearby, temporarily immobilized, creating an opening for you to aim at its weak spots and inflict a great deal of damage.
Additionally, you can also use Spike throwers at it to deal even more damage, but do remember to refrain from using any Purgewater, Acid, or Shock type attacks.
All in all, you need to keep moving and utilize the opportunities created and the machine’s large size in order to defeat the Slitherfang.
Search the Ruins for the Data
Defeating the Slitherfang has no importance in the Quest, it’s just a nuisance that you need to get rid of. The real deal is the rectangular building with a vault at its base just beside the tower structure.
Head over to the Vent Panel and open it, revealing a path that leads to the center of the building.
There should be a parkour course here. You need to climb over the rocks at your right onto a ledge, then head back over to the pipes and use them to reach the other side. You then need to move forward to the northeastern corner of the building to reach the Console. Interact with the Console to collect the data.
Find an Entrance to the Atbay Headquarters
The next objective is to follow Alva to the Atbay Headquarters through a path used by the soldiers and find an entrance to it. In the end, you should see a wall with an X embedded at the center. Pull out your handy pullcaster and pry open the gap with it.
Inside there should be a parkour course leading towards the top. You can use your focus to highlight the climbable spots and make your way up. Once at the top, you should see a beam that needs to be pulled down with your pullcaster. Cross the beam onto the platform ahead.
There should be a ladder to your lift, there’s a lock on it which you need to shoot first in order for it to fall down and be used. Climb the ladder to the top to find two Glinthawks resting atop the platform. You can either ignore them or kill them, it’s your choice.
Just to the left of the Glinthawks, there’s yet another series of walls that need to be scaled here. They’re pretty easy to scale, but if you’re still having trouble, you can use your focus to indicate where to go. Once you reach the top, you should see a beam leading to another part of the building. This part is blocked by some debris, but you can use your pullcaster to clear it out.
Unlock The Door
Inside this part of the building, you should see a locked console and some Datapoints. To unlock the console, you need to scan the Datapoints, and eventually, Alva will give you clues to figure out the passcode for the console, which is usually in the form of a date (MM/YYYY).
Putting in the right code will result in the door unlocking, leading to another room. In this room, there’s another door and an audio log on the table. You need to listen to the audio log first in order to unlock the door.
Climb up to the Ancestor’s Office
The door will lead to the edge of another platform. There is, yes, yet another climb from here. The first part of the climb will lead you to a beam that would fall, leading you rightward from your original spot.
Just ahead there’s a small opening that contains some Greenshine. Collect this Greenshine and then again continue your climb up the yellow handholds. Once at the top, you have to make a difficult jump from the corner brick handholds to a ledge on the other side of the building, so be careful!
At the top, you should see a beam leading to another opening in the building. Pull out your pullcaster to cling to the other side, then climb to the top. Do note that here, you often have to use your pullcaster to climb up, so keep an eye out for spots where you can use them.
Once at the top, you should see a path leading back inside the building near to the elevator shaft. There’s a Fireglam here that you need to ignite. Upon being ignited the Fireglam would explode, opening up the path to the top, so maintain your distance from it.
The Forbidden Legacy Door Code
From here, it’s just a simple climb to the top where Eileen’s Office is located. There should be a campfire located here which you can ignite, along with some artifacts and devices you can scan for clues. As you scan the items, Alva will steadily inform you of the clues to the Console’s Password.
The door code is 40, 2, 25, and 6. Just beyond this door is Eileen’s Console which contains all the information Alva needs, all you have to do now is interact with it.
The Forbidden Legacy Side Quest Rewards
This side quest offers a large amount of XP and skill points as a reward. Aside from the completion rewards, you’ll also find a lot of vaults and supply drops which also contain a decent amount of items.
As for the completion rewards, they include:
- 9750 XP
- 3 Skill Points
