so Nyx's powers are actually not dependent on himself. His magic is, in a way, on loan from the king in FFXV. Essentially, the only way the members of Kingsglaive can use their magic (who are worthy and capable of wielding the magic) is if the king is alive to wield the main power of the crystal (the thing that grants the magic). How would it work for Nyx in this case? I hope this makes sense I can provide actual examples from the movie if you need more details.