Uncanny X-Men, Vol. 1 #136 (1980)

Child of Light and Darkness!
Dark Phoenix returns to Earth and stops in at the home of the Greys. The X-Men come well prepared, with a combination of psychic shields, mental scramblers, and carefully orchestrated teamwork, and they still just barely manage to win out, due largely to Jean's help against her darker self. Just as they are settling down for explanations, they are teleported to places unknown.
Release Date: 1980