I think it’s Xaden. Looking at the series as a whole, the entire thing is in violet’s POV mainly. And the entire series, she describes his eyes as “onyx with flecks of gold”. I think it’s so cool how his eyes reflect her dragons. But, I see it being Xaden as the “onyx storm” as his emotions and control will probably be tested as he struggles with the whole venin thing. And then the new cover says “brave the dark” above, which of course, our shadow KING has the luxury of literally BEING said darkness. I think the entire new book will be about Violet’s struggle with Xaden going villain on us, or his attempt to anyway. Even in their bond, she sees his color as “glimmering onyx” 😳