After you saved Kiiran Dals' team from Remnant Stasis, she mentioned that Havarl was in decline. Though Havarl appears to be thriving, not everything is as it seems.
Acquisition
This mission is automatically received after completing Helping Havarl's Scientists in Pelaav Research Station.
Walkthrough
Talk To Kiiran Dals About Havarl's Problems
Kiiran Dals says that Havarl is having issues: uncontrolled mutations in the wildlife mean that the ecosystem is crumbling away. This appears to be an issue of having only two working monoliths. Assistant Torvar, one of the researchers, suggests asking the sages at Mithrava. Kiiran Dals scoffs at the idea, explaining how the sages are hermits that believe in isolating themselves from modern thought.
Find The Entrance To Mithrava, The Sanctuary
Regardless of what Kiiran says, Ryder will need to go to Mithrava. Head to the southernmost part of the ravine to find a Remnant door. Once through, continue all the way to the back of the room to find a Remnant console; it will appear out of the ground when Ryder approaches. Interface with the console to open a nearby wall.
Reach The Top Of Mithrava
Through the wall is a short corridor, at the end of which a pillar rises. Climb the pillar and head up through the hole. Use the gravity well to cross to Mithrava Ascent, evidently a tall Remnant tower. Use another gravity well to head up; once Ryder lands, go out the door.
Note: Along the way to Mithrava, there are several containers that can be looted.
The path forward is to the left of the exit; drop down from the ledge. The path appears to lead to a dead end, but interacting with a Remnant console brings down a platform. Use the pillars on either side to reach the platform, then jump from the platform to the pillars across the way. Continuing on will lead Ryder to another Remnant console; using the console will activate a bridge for Ryder to cross. There are a few Remnant on the other side, so be mindful of the edge while fighting.
At the end of this ledge is a short pillar for Ryder to jump on to reach the next level. Keep moving until Ryder comes to yet another pillar. Climb the pillar to reach a Remnant console. Interfacing with the console activates another bridge. Cross the bridge and climb up a few consecutive pillars and ledges to reach a Remnant door, which opens to another gravity well that takes Ryder upwards and finally to Mithrava itself.
First Sage Esmus comes to ask how and why Ryder is here. However, once informed, he says that restoring Havarl is impossible, because they do not have the knowledge of the third monolith or the vault. It is explained that angara believe in reincarnation, and that sometimes memories of a soul can be remembered via familiar items. Zorai, an ancient champion of the angara, was said to have knowledge of the Remnant; however, the soul has been reborn in Taavos, a member of the Roekaar. Esmus gives Ryder the location of an item that belonged to Zorai.
Before leaving the area, Ryder can grab a memory trigger for Ryder Family Secrets. Ryder can also pick up Task: Unearthed from Sage Fleera and A Lost Sister from Sage Amurd.
Note: Don't forget to deploy the Forward Station before leaving.
Go To The Location Esmus Mentioned
Heading to the navpoint on the Havarl map at the Remnant Abyss, Ryder will run into Remnant and likely some hostile wildlife as well. Past the hostile, Ryder will find a large Remnant structure, and SAM will notify when Ryder is close to the location Esmus provided. More Remnant will be here, as well as a Galorn, though the latter can be avoided if careful.
Locate Zorai's Heirloom
Enter the Remnant door and take the gravity well down to the Remnant Abyss. Moving forward, Ryder will find a Remnant console on a small island platform. Interacting with the console causes all three walls of the room to come down. Remnant will attack from the left wall from the console, so be prepared.
The path the Remnant came from is a dead end, and the path straight ahead is blocked by a barrier and a gap, so head to the path from the right of the console. Be ready for more Remnant just around the corner. Continue until Ryder reaches a door, which leads to the area north of the Remnant console Ryder interfaced with earlier; here Ryder will find a Destroyer alongside a few other Remnant.
Once the Remnant have been defeated, Ryder can find a Remnant console to interact with on the eastern side of the room. The console activates a bridge to the small separate room on the map, which has a container and an Adaptive Remnant Core Device (+100 
). Continue north through the door until Ryder reaches a room with a giant pillar in the middle, surrounded by yet more Remnant, including two Nullifiers.
Once cleared, interact with the door towards the marker to reveal ancient angaran remains alongside a data drive. Searching the remains will give Ryder Zorai's Heirloom, apparently a gauntlet of some sort. Examine the data drive for more information.
There is no need to backtrack from here. Instead, go to the side room and interface with the Remnant console, which both opens up the other wall for a shortcut back to the entrance and creates platforms on the pillar in the center. Ryder can climb up the pillar for two containers before returning to the surface.
Go to Taavos' Roekaar Camp
Head to the marker near the Chasm of the Builders. There are Roekaar there, but they have activated the bridge, allowing Ryder to cross once they are defeated. Scanning the bridge awards +10 
More Roekaar are waiting across the bridge, so be prepared. After clearing them out, go up the stairs.
There is a datapad for Task: Roekaar Manifestos on one of the crates here. Go through the door to the Ancient Courtyard, where a Forward Station will activate.
Moving forward towards the marker will activate a Roekaar shuttle dropping in enemies. Once they are defeated, head towards the camp which located in Old Pelaav.
Defeat The Roekaar At Their Camp
As can be expected, the camp is heavily populated by Roekaar, as well as a few Challyrion. After clearing out the front, walk up the stairs to find the main encampment. More Roekaar are here, accompanied by Adhi.
Find Taavos
Once all enemies have been defeated, Ryder can search the two empty buildings for Roekaar manifestos, one in each. The Archive System required to complete A Lost Sister is also found in this area.
Taavos is ready to die, but Ryder instead convinces him to hold Zorai's gauntlet. He suddenly gains knowledge of an underground monolith, along with emotions not his own. He reluctantly agrees to meet Ryder at given coordinates and lead the squad to the monolith.
Meet Taavos At The Entrance To The Lost Monolith
Head to the northern end of the ravine and talk to Taavos. With his newfound memories, he is able to activate the Remnant technology to enter the Buried Cave where the monolith resides.
Walk with Taavos down the path; if Ryder takes the time to look around, he'll stop and wait for Ryder to catch up. Urged by his memories, he takes a side path that leads to the remains of another ancient angara, along with an audio log.
Examine The Remains
The remains turn out to belong to Zorai. Scans and the audio log reveal that she was female, something the sages evidently forgot.
Continue With Taavos To The Monolith
Continue down the path with Taavos. He leads Ryder to a gravity well, with the third monolith further down and away.
Activate Havarl's Third Monolith
Once Ryder has landed, there's a Remnant console just ahead. Interface with the console to activate this monolith.
Taavos says he will stay here, as he has much to think about. With all three monoliths active, Ryder can now head to Havarl's vault.
Before leaving, collect the Memory Trigger. On a ledge to the north east in the area Ryder scan an Adaptive Remnant Core Device for +100 .
Find And Activate Havarl's Vault
Head to the newly revealed vault near Old Pelaav at the Ancient Courtyard. There are a few Remnant near the entrance. Clear out the Remnant and go in the vault doors. Use the Gravity Well.
Turn On Emergency Power
Ryder will need to turn on emergency power by interacting with the power console, though Remnant spawn and attack Ryder the moment the power turns on.
Lift The Lockdown On The Vault
On the high ledge is a turret. Activate the turret with the nearby console, and it will assist Ryder during the escape from the vault. There is +200 close to the turret.
Jump down and discover a container with remtech, which will be accessible during the escape from the vault. SAM marks it on the map.
Several Remnant consoles are now available to interface with, although Ryder must activate them in a certain order to proceed. Facing the door that Ryder is trying to open, use the left console closest to the door, the left console further from the door, the right console closest to the door, and the right console further from the door.
The purification activation console is directly through the door; however, before starting the purge, go around to the other side to find another console to interact with. Solve the glyph puzzle presented in order to receive 2 skill points.
Escape The Purification Field And Exit The Vault
When ready, activate the purification console and run, although Ryder can't run far, as the door closes on and Ryder has to fight off Remnant while the purification cloud approaches. The doors will open in the nick of time but there are more Remnant in the next room as well. Ryder doesn't have to fight them all if the Remnant turret was activated at the start. When the door opens Ryder receive +367 XP (an odd quantity, but it's not from killing Remnants).
Run to the container on the lower floor behind the turret and collect the remtech.
Use the console on the upper level to stop the purge and successfully activate the vault.
Note: The Fusion Mod of Robotics is awarded for lifting the vault lockdown.
Ryder receives +1330 XP, +900 AVP 
, +55% Havarl viability (the XP doesn't show on screen).
Return to Kiiran Dals
Report in to Kiiran Dals at the Pelaav Research Station. Esmus and Taavos are there as well, having been influenced by Ryder's actions. While no outpost can be placed on Havarl, Kiiran Dals invites people from the Initiative to help the research station.
Aftermath
Ryder will receive the email "Harval: Science Team Deployment" once the mission is completed and has returned to the Tempest.
Acknowledge the email to receive the mission Cross-Cultural Alliances.
Once this mission is complete, Ryder can now access a well hidden decryption puzzle (needed for the Cryptographer Achievement) and container. The container awards the Fusion Mod of Adrenaline. Refer here for detailed instructions on locating the puzzle and Fusion Mod.
A terminal in Mithrava will have the following entry after the mission is complete.
Rewards
- +1330 XP
- +73 AVP
- +5% Havarl viability