One word kill

In January of 1986, Nick Hayes, a fifteen-year-old genius, discovers he has what is probably a terminal illness. At the same time, an even stranger thing happens--a girl, Mia, joins his Dungeons & Dragons-playing friend group, and immediately Nick notices a strangely familiar man with should-be-impossible abilities following him. The man warns Nick that Mia's life is in mortal danger, and only Nick can save her.