Published Jun 1978 by Marvel.
1st full appearance of Mystique.
Dave Cockrum cover (pencils & inks). Chris Claremont script. Jim Mooney & Ricardo Villamonte art. "The St. Valentine's Day Avengers Massacre" Ms. Marvel and the Avengers (Wonder Man, Vision, Scarlet Witch, YellowJacket, & the Wasp) vs. Centurion and Mystique. First *full* appearance of X-Men Character, Mystique.