Mask of the Revenant

"Built to the Unum's specifications, and fitted with prismatic lenses supplied by the Quill Onkko."

- In-game Description

Mask of the Lost One is a decorative mask for the Operator that doubles as the trigger for a mini-quest. The quest itself tells the tale of a warframe left behind to prevent the return of the Sentient Eidolon, and upon completion: rewards the blueprint for the Revenant warframe.

The mask itself can only be purchased from the Cetus vendor Nakak once the players have achieved the Observer rank with The Quills, and has talked to her with the "Mask?" prompt.

Walkthrough
Nakak will inform the player that she received a strange mask from Onkko. She will sell the Mask of the Lost One for 1 Cetus Wisp.

The player must equip the Mask of the Lost One on their Operator and head into the Plains of Eidolon at night. They must then follow the marked white waypoint at the southwest side of Gara Toht Lake. Spotting a large spectral being that vanishes, the player must look around in Operator mode for a small blotch of energy with a hand reaching out from it. Nakak will then receive a vision from the Unum, explaining that the lake is where Gara had felled the Sentient, and a warden was placed to prevent the Sentient from resurfacing until the warden was lost.

Nakak will ask the player to rebuild "the lost one", mentioning that the Grineer have found relics and suggests talking to Konzu. The player must now initiate a Bounty until they find and build a part of the Revenant warframe.

If the player already has one piece of the Revenant warframe, Nakak will tell the player to proceed.

The player once again must return to the Plains of Eidolon. At the northeast side of Gara Toht Lake is where the specter appears, and the player must interact with another small blotch of energy. Nakak sees another vision from the Unum, mentioning that the warden continued to strike down the Sentient until it briefly hesitated at one point.

To proceed, the player must return to Bounties to find and build another part of the Revenant warframe.

The final blotch of Sentient energy resides on a river between Gara Toht Lake and Renthi Spring. Nakak receives one final vision where the lost one extended a hand to the Eidolon, allowing it to emerge and drag the warframe into it place. Just as Nakak finishes the tale, a Revenant Specter appears and begins attacking. Nakak tells the player to defeat the specter that serves as the mask for the Eidolon. Defeating the Revenant Specter will yield the blueprint for the Revenant Warframe.

Mask?
Nakak: Sales! I really love it. Look, to be honest - Onkko made this mask, and it's a bit of a creep piece. He set some weird eyes in it, said "The Unum chooses!" Ai-yo. 'Unum chooses' is short for nightmares and a wild Vobi hunt. Funny, right? Tower spirit choosing me... and I can't stand heights! I think that makes me 'fake-Ostron'? Tower work was more my cousin, Mukha's calling, he's fearless... uh, was fearless. Well... Buy the mask, Tenno! Or don't, up to you.

Nakak: Great. Onkko's note here says "Wear the mask when the moon shines bright... upon the haunted Plains of Eidolon. You could do that. OR you could take the day off, toss that mask into the lake. Oh! Sorry Onkko, we lost it!

A New Threat Looms
Nakak: Alright. Fine, Tenno. But if I start seeing Unum things... it's on you!

Nakak: Chut! Is it working? Ai yoooooo, it is working. No getting out of this now, is there?

Nakak: What is that? Some Eidolon? No. Wait. Somehow, it's not. I can't belive I'm saying this but... can you get a closer look?

Nakak: Why do I remember this? The land here, killing hot, full of strangeness. This is where Gara vanquished the Sentient! Where its essence fell into the ground, trapped in some unworldly between. It wanted out. At moon rise its ghostly tendrils would emerge, seeking form, seeking... return... And this... this was the warden... a Warframe. I can see it, like a dream repeating... The tendril emerging night after night, and the warden, night after night, cutting it down... until the warden himself was lost. How?

(If the player does not have a Revenant part)

Nakak: Khanung. The Unum is in my head. She commands you to seek the pieces of the lost one... and rebuild it. What? Like I have time for this... I've got a business to run! Ai yo... Onkko said Grineer Command was excited about some relics they'd dug up. Maybe you should talk to Konzu?

A Terror Long Forgotten
Nakak: The lost one's spirit calls for aid! We can't just leave him... wherever he is. Are you ready, Tenno?

Nakak: Khanung! Do you see? The lost one struggling... what happened to him?

Nakak: Ugh... here we go again. I remember... the pool glows, the tendril emerges... and this Warframe draws its weapon to destroy yet again... wait... no. He does not strike this time, he watches. Why? The phantom tendril grips trees and rocks, probing for substance... then fear returns and the lost one slashes the phantom, sending it back to its purgatory...

The Lost One
Nakak: I... I need to know what happened to the lost one. Find the place where he calls out to us. Let me know when you... when you're there.

Nakak: The lost one's spirit calls for aid! We can't just leave him... wherever he is. Are you ready, Tenno? There! See? He's further out... trying to escape... can you help?

Nakak: I see... the tendril once again. Instead of striking it... the lost one... he extends a hand to the Eidolon?! Khanung! What was he thinking? He's like Mukha on the tower. Every day, Mukha clamored across the tower ledges and scaffolds - nothing happened, day in and day out... Routine took his fear away. Uhh... Tenno - I see the tendril has him now. The lost one. He's become an anchor for the Eidolon - to pull itself onto our world! The lost one struggles, pulling back, but it's too late! The Eidolon begins to emerge. But... the lost one realizes - he stops... he lets go... And so he falls. Falls from this world of the living and down, down, down into the next. Mukha, that so was[sic] stupid.

Nakak: Khanung! It's attacking! Hold on, this isn't some trapped Warframe spirit! He's become a mask for some Eidolon! We can't let it rebuild itself! Tenno, you need to destroy it!

Nakak: The Eidolon's violence is within him, Tenno! Cast it out! Free him!

Nakak: Thank the Unum. You've driven out the Eidolon... but what's left I wonder?

Nakak: Mukha, the warden... they lost their fear. But I have plenty... and... I'm still here. Is that why the Unum chose me for this? Thanks for the nightmares! Glad that's over. If you need me I'll be under nana's blanket with a flask of hot chimurr. Maybe I'll burn some incense, for Mukha. Y'know, sales isn't so bad after all. Everyone needs a mask, right?

Trivia

 * Unlike other quests, this quest is not picked up in the Codex.