User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-4449950-20160926123003/@comment-5303512-20160926125907

Well Gray's Ice Devil Slayer is a Curse so it must be feeding on his hate for END to fester and grow, so it would be no surprise that it has some sort of control over him. And Nats is (more or less) controlled by END essence.