Xorn was introduced as the future version of the young, time-traveling Jean Grey. She allegedly could not remove her helmet for extended periods without becoming unstable and dispersing into pure psionic energy. A later version of Xorn, however, seemed to be nothing more than a psychic construct animated by Xavier II.
Xorn was seemingly destroyed in X-Men: Battle of the Atom #2. However, Xavier II sent a note to himself in the future to prevent the Brotherhood from coming back for that first mission, allowing Xorn to appear in subsequent stories.
Magneto confronted the Brotherhood in the timestream in X-Men: Blue #20, implying they did not survive the encounter.