Niemand
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There's sarcasm in his eyes, a smirk. "You're nobody, you're just a pawn" (c) Stas Boretsky.
So, who is the Black King then? Morning? It's quite possible, judging by his voice. As well as Arandor. On the other hand, he admitted that he is the Black figure. Plus, if the Rooks didn't go to him, that's an epic fail, as he revealed himself anyway.
I'm not arguing that I also failed. However, I at least tried to solve the hint and acted consistently.
I believe the theory that the Rooks went to Arandor-King, but didn't kill him because of immunity, has merit. That's why I'm voting for it. If there are alternatives, suggest them.
And you believe Arandor's confession?
If we're going to execute a king, it's better to execute Morning. There's a 50% chance that we'll hang the Black King.