User blog comment:SayuriDarling/The Sword of Summer Official Discussion/@comment-5984859-20150804101842/@comment-5471249-20150806051425

We don't yet know enough to make such a confirmation - maybe it seemed to him that he had died, while he had in fact only been transported to Niflheim... Also, even if Magnus is somehow killed in the first book, that only implies that he'll be somehow resurrected some time later.