Show Daario to Daenerys:
DAARIO: Everyone has a choice. Even slaves have a choice. Death or slavery.
DAENERYS: So what else can I do?
DAARIO: Marry me instead.
DAENERYS: Even if I wanted to do such an inadviseable thing, I couldn’t.
DAARIO: Why not? You’re our queen, you can do as you like.
DAENERYS: No. I can’t
DAARIO: Then you are the only person in Mereen who’s not free.
Daenerys sits up in bed.
The Night’s King is a king, he can do as he likes. Or is he just another one of the unpeople who aren’t free?
In this brilliant reddit thread “Mr_Bricksss” wrote how how the Night’s King’s broach looks like a (blood)raven skull. And a cylindrical tower.
The post went on to mention that it also has his “sigil” of The Nightfort on it. But I don’t think it is his sigil. This is not the equivalent of an Other direwolf on his chest. I think it is a “ruby.”
Rattleshirt sat scratching at the manacle on his wrist with a cracked yellow fingernail. Brown stubble covered his sunken cheeks and receding chin, and strands of dirty hair hung across his eyes. “Here he comes,” he said when he saw Jon, “the brave boy who slew Mance Rayder when he was caged and bound.” The big square-cut gem that adorned his iron cuff glimmered redly. “Do you like my ruby, Snow? A token o’ love from Lady Red.”
... Melisandre spoke softly in a strange tongue. The ruby at her throat throbbed slowly, and Jon saw that the smaller stone on Rattleshirt’s wrist was brightening and darkening as well. “So long as he wears the gem he is bound to me, blood and soul,” the red priestess said. “This man will serve you faithfully. The flames do not lie, Lord Snow.”
This seems very similar to a different magic on the other side of the world. Daenerys in The House of The Undying:
A long stone table filled this room. Above it floated a human heart, swollen and blue with corruption, yet still alive. It beat, a deep ponderous throb of sound, and each pulse sent out a wash of indigo light. The figures around the table were no more than blue shadows. As Dany walked to the empty chair at the foot of the table, they did not stir, nor speak, nor turn to face her. There was no sound but the slow, deep beat of the rotting heart.
… Her own heart was beating in unison to the one that floated before her front of her, blue and corrupt.
Someone or something else is also very corrupt. And Bran is in under his control.