X-Men: What Happened To Everyone Who Became The Phoenix?
13. Lady Phoenix
Not a lot is known about the Phoenix of 1,000,000 B.C except that she was the first human to ever have possession of the Phoenix Force and teamed up with Avengers of that time to fight, and ultimately defeat, a Celestial known as the Fallen.
No-one knows what eventually happened to her as she seemed to vanish off the face of the Earth. Expect more answers to come later in Jason Aaron's Avengers comic.