Before it reached its cosmic heights, Chris Claremont and John Byrne's "The Dark Phoenix Saga," one of the most famous and beloved X-Men stories of all time, began with the Hellfire Club finding a new member in someone who appeared to be Jean Grey. While the real Jean was sitting in stasis after her fateful encounter with the Phoenix Force, a Phoenix-created duplicate of Jean with her memories and personality that embarked on a fall from grace that saw her become the Black Queen of the Hellfire Club.
When the Phoenix first took Jean's form, she was left in quite a vulnerable position. For one thing, Jean's exponential increase in power levels left her telepathic powers unstable. Indeed, the Phoenix Force proved difficult to control, often forcing this Jean to succumb to her baser, violent urges. On top of this instability, Jean had recently endured a climactic battle against Magneto, after which the other X-Men were presumed dead.
By isolating this Jean from her boyfriend, Cyclops and her other loved ones, the Phoenix was vulnerable to sinister machinations, like those of the Hellfire Club's Mastermind, a mutant illusionist and former member of The Brotherhood of Evil Mutants. Starting with Claremont and Byrne's X-Men #129 in 1980, Mastermind was tasked with corrupting Jean into the Black Queen, so that he could become part of the Hellfire Club's Inner Circle.
With the help of Emma Frost's White Queen, Mastermind toyed with Jean's mind for months. The illusionist made Jean believe that he was a dashing gentleman from the 18th century and that she was his lover, "Lady Grey." At times, Jean would be in the middle of a mission with her fellow X-Men, when one of Mastermind's illusions reappeared. Jean and Mastermind would be back in the 18th century, engaging in soothing, leisurely activities as old lovers. These illusions were strategically planted in Jean's mind on-and-off for a considerable period of time before Mastermind made his final play.
When the X-Men investigated the Hellfire Club's headquarters, Mastermind used his previous influence to gain full control of Jean's mind. This time, Mastermind encouraged the Phoenix to give in to her darker desires. "Lady Grey" took over, finally becoming the nefarious Black Queen. The Phoenix then turned on her fellow X-Men, knocking Cyclops unconscious. Jean, corrupted into the Black Queen, stood by as the rest of The Hellfire Club defeated and captured her teammates. Mastermind seemingly accomplished his goal, transforming Jean into his Black Queen and becoming part of the Hellfire Club's Inner Circle.
Despite the pleas of her fellow X-Men, Jean was past the point of no return. Due to Mastermind's illusions, all the Phoenix could see was an 18th-century version of the events transpiring around her. The X-Men were merely soldiers of the Revolutionary War whom the Hellfire Club had captured.
Nevertheless, Cyclops refused to give up on his love, attempting to reach Jean through their psychic rapport. Finally, in a psychic duel, Mastermind "killed" Cyclops. The shock of Scott Summers' "death" triggered an intense emotional reaction within the Phoenix, causing her to remember who she was.
Jean used her power to free the X-Men, allowing them to strike back at the Hellfire Club's Inner Circle. Although her memories had returned, Jean held a colder, more ruthless demeanor. Confronting Mastermind, the Phoenix overloaded his mind with more knowledge than the human brain can comprehend. As a result, all the villainous mutant could do was sink into a catatonic state. By the time Cyclops realized that something was wrong with Jean, it was too late. As the X-Men escaped the Hellfire Club's lair in their jet, Jean transformed once more. This time, the darkness completely overcame the X-Man, morphing her into the Dark Phoenix. The malevolent power which Mastermind had unlocked within The Phoenix finally came free.
Without Mastermind's corrupting influence transforming her into the Black Queen, this Jean Grey would never have become the Dark Phoenix in the first place, and the real Jean Grey may very well have stayed in stasis at the bottom of Jamaica Bay forever.