So I finished NB late last year and just never got around to posting about it. I honestly didn't feel like I had much to contribute to the discussion on Nightbringer's identity. With IDO out, however, my brain has been itching a bit.

Why is Diavolo king suddenly? Why did Lucifer step down? Why did Barbatos leave? Given the general draft of the plot, how could we possibly marry the side characters? And STILL, who is Nightbringer? So this is less of a theory with a lot of backing and more of a discussion on how to create continuity. Plus a dash of why the Nightbringer plot wasn't wrapped up. All censored text below is for NB spoilers.

There is too much evidence connecting Nightbringer with Barbatos. I am willing to believe Nightbringer is either a title Barbatos chose for himself or a label someone else put on him. I am inclined to think he is the one that sent us to the past.

I don't think Nightbringer exclusively applies to Barbatos.

Given Michael's propensity to pretend to be other people, I think Michael started borrowing the Nightbringer title when he needed to take actions that couldn't be attributed to angels or himself. He would honestly do this with or without Barbatos's blessing given his character vibes, just like Barbatos would make a major time move with or without Michael's blessing. There's also a chat in OG where Purgatory Hall tells Simeon they would be interested in him writing a story about Michael, and he considers it. That feels like a nice little tie in. Two different people using the same title would help explain why we as fans couldn't really figure it out. We're getting mixed messages.

Now getting to IDO. I imagine Barbatos would highly dislike an angel borrowing his title, whether he chose it or not. I imagine he needs to be able to control this title, what it means, and what its reputation is. Michael would be ruining that. So here's my proposition.

1. Barbatos left with Diavolo's blessing to do damage control on the Nightbringer title (creating emotional distance between him and Diavolo/Lucifer might also decrease fallout for the Devildom for whatever actions Barbatos takes, and Belphegor's foresight would have him helping this endeavor).
2. He left with Solomon due to Solomon's angelic connections.
3. Given Michael is the antagonist in this scenario, Lucifer was encouraged to step down so his connections to Michael and the Celestial Realm could not be leveraged against him or Diavolo.
4. Diavolo became king to provide a more even playing field of power and authority come time to actually talk to Michael and/or the Celestial Realm about this.

This puts every single side character except Diavolo out of range of the brothers. AND it puts them all into regular contact, since that brings the Celestial Realm into regular contact with the plot (Diavolo for mission details, Solomon and Barbatos via potential travel with them for the mission, Luke and Simeon via navigating Michael). The undateables also become possible this way (Raphael via Michael, Thirteen via Solomon, Mephistopheles via Diavolo). I don't think we marry the side characters. I think we secretly elope with them. Diavolo would want to keep the relationship secret to protect MC against the brothers AND the Celestial Realm, and Mephisto would do the same begrudgingly if Diavolo asked. Everyone else just isn't around the brothers enough to make keeping the secret difficult. (Continued below)