Revenant fragments
The Revenant fragments are found during the Mask of the Revenant mini-quest. Each contains a hidden fragment of narration by Nakak, heard as dialogue during said quest. The hidden dialogue is listed last.
A New Threat Looms

A vision comes. Something new is stirring within the ground. Seeking a body to call its own, longing to strike those who sent it to the grave.
A Terror Long Forgotten

A fragment of the ancient Sentient that once terrorised these plains.
The Lost One

Once the warden of the Plains, tasked with protecting the Unum's Tower and striking down the ghostly Eidolon forms that rose from the depths every night. Night after night, he cut them down, not allowing them to take form. Then, one night, he disappeared, lostโฆ forever.
Revenant (hidden)

[1] A New Threat Looms:
"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 moonrise its ghostly tendrils would emerge, seeking form, seekingโฆ return. And thisโฆ this was the wardenโฆ a warframe. I can see it, like a dream repeatingโฆ. The tendril emerging, night after night, and the warden, night after night, cutting it downโฆ until the warden himself was lost. How?"
[2] A Terror Long Forgotten:
"Ughโฆ here we go again. I rememberโฆ. The pool glows, the tendril emergesโฆ and this warframe draws its weapon to destroy yet againโฆ waitโฆ no. He does not strike this time, he watches. Why? The phantom tendril grips trees and rocks, probing for substanceโฆ then fear returns and the lost one slashes the phantom, sending it back to its purgatoryโฆ."
[3] The Lost One:
"I seeโฆ the tendril once again. Instead of striking itโฆ the lost oneโฆ 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 โ nothing happened, day in and day outโฆ. Routine took his fear away. Uhhโฆ Tennoโฆ I see the tendril has him now. The lost one. He's become an anchor for the Eidolon โ to pull itself onto our world! The lost one struggles, pulling back, but it's too late! The Eidolon begins to emerge. Butโฆ the lost one realises โ he stopsโฆ he lets goโฆ and so he falls. Falls from this world of the living and down, down, down into the next. Mukha, that was so stupid."