in the vacancy, a revamped Jean Grey soon rejoined the X-Men under her new persona of "Phoenix". Angel, Beast, Iceman, Havok, and Polaris also made significant guest appearances. The revived series was illustrated by Cockrum, and later by John Byrne, and written by Chris Claremont. Claremont became the series' longest-running contributor. The run met with critical acclaim and produced such landmark storylines as the death of Thunderbird, the emergence of Phoenix, the saga of the Starjammers and the M'Kraan Crystal, the introduction of Alpha Flight and the Proteus saga. Other characters introduced during this time include Amanda Sefton, Mystique, and Moira MacTaggert, with her genetic research facility on Muir Island. The 1980s began with the comic's best-known story arc, the Dark Phoenix Saga, which saw Phoenix manipulated by the illusionist Mastermind and becoming corrupted with an overwhelming lust for power and destruction as the evil Dark Phoenix. Other important storylines included Days of Future Past, the saga of Deathbird and the Brood, the discovery of the Morlocks, the invasion of the Dire Wraiths and The Trial of Magneto!, as well as X-Men: God Loves, Man Kills, the partial inspiration for 20th Century Fox's movie X2: X-Men United, which was released on May 2, 2003. By the early 1980s, X-Men was Marvel's top-selling comic title. Its sales were such that distributors and retailers began using an "X-Men index", rating each comic book publication by how many orders it garnered compared to that month's issue of X-Men. The growing popularity of Uncanny X-Men and the rise of comic book specialty stores led to the introduction of a number of ongoing spin-off series nicknamed "X-Books." The first of these was The New Mutants, soon followed by Alpha Flight, X-Factor, Excalibur, and a solo Wolverine title. When Claremont conceived a story arc, the Mutant Massacre, which was too long to run in the monthly X-Men, editor Louise Simonson decided to have it overlap into several X-Books. The story was a major financial success, and when the later Fall of the Mutants was similarly successful, the marketing department declared that the X-Men lineup would hold such crossovers annually. Throughout the decade, Uncanny X-Men was written solely by Chris Claremont, and illustrated for long runs by John Byrne, Dave Cockrum, Paul Smith, John Romita Jr., and Marc Silvestri. Additions to the X-Men during this time were Kitty Pryde/Shadowcat, Rogue, Jean Grey/Phoenix, Psylocke, Dazzler, Longshot, Jubilee, Forge