With the new experience, of a cosmic entity dealing with human emotions, Phoenix was thus an easy target for the illusionist, Mastermind, who needed a trophy to join the Inner Circle of the prestigious (and infamous) Hellfire Club. With the help of a psionic device, created by the club's White Queen, Emma Frost, Mastermind made Phoenix believe that she was his 18th-century lover, Lady Grey. Phoenix joined the club as their Black Queen, which would allow her to relish the extremes of human emotion and sensuality. When the X-Men came to her rescue, Jean's true love, Cyclops, faced Mastermind in a psychic duel and was defeated. Cyclops' "death" broke Mastermind's hold over Phoenix's psyche--as well as any control Jean's psyche had had over the Phoenix. It brushed aside the X-Men and departed for the heavens.
As the Black Queen
As the Phoenix was the light and life of the universe, the Dark Phoenix represented power and destruction. The Phoenix became Dark Phoenix from allowing human emotions to cloud its judgment. In this state, Phoenix was the strongest, but also an evil entity that thirsted for power and destruction. Totally uncontrollable, Dark Phoenix was a force to be reckoned with as it was not bound by a human conscience. When Dark Phoenix flew back into space, her true firebird form was seen across the entire universe and even attracted the attention of Eternity.
To sate its hunger, the Dark Phoenix flew to a distant galaxy and devoured the star of the D'Bari system. The resulting nova killed billions of innocent people and a Shi'ar vessel attacked it. The Shi'ar Empress, Lilandra Neramani, was alerted before the vessel was destroyed and called a council of her intergalactic counterparts, including the Kree and Skrull empires. They concluded that, for the rest of the universe to live, Dark Phoenix must die.
Meanwhile, the Beast had found a way to neutralize Phoenix's power, temporarily. When Dark Phoenix returned to Earth, the X-Men held it at bay, while Professor X rebuilt the mental blocks in Phoenix's mind, that Mastermind had broken down. Scott, then, proposed to the Phoenix. At that moment, the Shi'ar arrived and teleported everyone aboard their flagship. Lilandra announced that Phoenix, who everyone believed to be Jean, would have to die. Professor X, unwilling to give up on one of his pupils, but romantically involved with Lilandra, challenged her to Arin'n Haelar, a Shi'ar duel of honor that cannot be refused. Professor X named the X-Men as his champions in the duel, Lilandra, her Imperial Guard.
Cyclops and Phoenix
The duel took place the next day, in the Blue Area of the moon. The Imperial Guard made short work of the X-Men, leaving Cyclops and Phoenix to make a final stand. A stray bolt hit Cyclops and Jean's control broke and Phoenix became the Dark Phoenix again. Lilandra ordered Plan Omega, which would destroy the whole solar system, in hopes of eliminating Dark Phoenix as well. Professor X ordered the X-Men to subdue Phoenix, so that Plan Omega would not be necessary, and they managed to subdue the Phoenix, for a brief time. In this moment of lucidity, Phoenix fled into a back alley, with Cyclops in pursuit. Phoenix subdued him, with her telekinesis, and told him that she was losing control of herself and that there was only one way to prevent the Dark Phoenix from taking dominance over her again. Unbeknown to Cyclops, as Phoenix was talking to him, she was using her powers to activate an ancient Kree energy weapon behind her. Phoenix, then, bade Cyclops an emotional farewell and triggered the weapon, which atomized her on the spot.