We pick up shortly after The War Within, with your operator being called by a strange signal from an abandoned Grineer ship. This quest focuses on Rell, a Tenno cast out from the group by Margulis for blinding her and left out of the Second Dream. Privy to the existence of some being titled "The Man in the Wall", Rell formed the Red Veil syndicate to fight against his influence on the system and reality at large. We're unsure of what this being is, or what they want, but they don't seem to be friendly. Without the grace of Cryosleep, Rell was forced to completely merge his consciousness with his warframe. 

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With the influence of the Man in the Wall creeping into our reality through Rell's connection to the void, Rell sacrificed himself to keep it bay. He chose to bind himself into a forced prison where he would rest in eternal damnation and torment, a victim of the Man in the Wall's musings and torture to keep him out of our reality. Working closely alongside the Red Veil through this quest, you are led to a full showdown in Rell's eternal prison. You succeed in freeing Rell from his servitude, finally allowing him to rest. In turn, however, there is no vigil to keep the Man in the Wall at bay, and it is implied that his influence will begin to seep into our reality going forward.