The “Dark Phoenix Saga” in 1980 led to a change in the line-up of the team, with the death of Phoenix (Jean Grey), and Cyclops leaving the team to mourn for her. Comics writers and historians Roy Thomas and Peter Sanderson observed that “‘The Dark Phoenix Saga’ is to Claremont and Byrne what ‘the Galactus Trilogy‘ is to Stan Lee and Jack Kirby. It is a landmark in Marvel history, showcasing its creators’ work at the height of their abilities.” The storyline also saw the introduction of recurring antagonists the Hellfire Club, and its Inner Circle consisting of Sebastian Shaw, Emma Frost, Harry Leland, Donald Pierce, along with Mastermind, previously a member of Magneto’s Brotherhood. The new teenage mutant Kitty Pryde was introduced in #129 (Jan. 1980) and joined the X-Men in #139. The Dazzler, a disco-singing, roller-skating mutant, was introduced in #130 (Feb. 1980), but did not join the team, instead headlining her own solo title.
A new Brotherhood of Evil Mutants, led by Mystique, was introduced in the “Days of Future Past” storyline (#141-#142, Jan–Feb 1981) in which a time-travelling Kitty Pryde tried to avert a dystopian future caused by the Brotherhood assassinating Presidential candidate Senator Robert Kelly. Byrne plotted the story wanting to depict the Sentinels as a genuine threat to the existence of the mutant race. He then left the series after #143, being replaced by a returning Cockrum, who in turn was succeeded by Paul Smith and John Romita Jr.