I was reading the story again but in Jin's perspective or parallel universe and found more evidence that Nathan is Tenebris.
-Jin asked Nathan if he was a Soul Worker because he seemed like someone who would be, then he denied it because he did not have the same energy that Maverick gave off. However, he also said that it was not too far off.
-The the detail that gave it away was when he started describing what seemed to be Rosca. He said "Her back lit under the sunlight and looked like she had absolute power." If I remember correctly, he also said that she came down from the heaven to judge him which could be describing how she appeared in front of him, "It is a pillar of light that extends to the sky that reflects in the vision of the eye. It appears without a predecessor, without sound, without meaning, as if it had existed from the beginning"
But there's one thing I have not mentioned(I think). When Kent "The world will end when the black dragon soars the skies" As far as I know we have yet to have any information about this dragon that Kent once mentioned, but for him to mention it(A God acknowledging the danger of such creature) seems to be quite threatening, perhaps the last boss?
Never played Jin's story so can't comment on that.
Nathan doesn't seem to be corrupted in the same way Ladi was though.
And he saw Rosca's back? Shouldn't she be facing towards them if she was attacking them?
Well, we don't exactly know Tenebris's side of the story. So it might be a possibility. Plus lets not forget that Tenebris was close to Maverick who also saw her back.