@Kampfarsch
I think what it means is that a child that is have human and half spirit can not be born, however becoming partially spirit, with extraordinary circumstances(like the QUEEN of spirits falling in love with and marrying you) is possible, and since both parents are spirit, they can have a child.
@Aslinn
Yes but the king has no right to demand such a thing. He is essentially asking for favours from people who can destroy him but don't due to having morals, and then tries to bully them into letting him exploit them just because he wants to. The spirits, greater beings than him, grant those blessings because they want to, and otherwise don't meddle as they have no reason to, hasn't he learned that angering them will get him cursed? And he's already cursed? His greed knows no bounds. And even if there is a reason, since it is a human problem, the solution should be human too, how dare he try to involve an outsider that wants nothing to do with him in his personal matters. The current generation(the king), is essentially just as bad, as they too, desire the same thing, and will stop at nothing to get it, despite knowing why, and knowing the consequences.