From the Marvel Encyclopedia:
"In an alternate timeline ruled by Apocalypse, Mr. Sinister created Nate Grey from genetic material from Cyclops and Jean Grey. X-Man eventually escaped to the X-Men's reality to fully realize the promise of his powers. He became a modern-day mutant shaman, existing to heal and guide all peoples, loving them without question. He fought to protect the planet from the incomprehensible, impossible threats of parallel Earths strung along a spiral from one end of reality to another. It was to save humanity from destruction at the hands of such a threat that he sacrificed himself."