The character Dark Phoenix was created as an alias for Jean Grey in the Marvel Comics universe, introduced as part of the Phoenix Saga storyline.
Dark Phoenix made her first comic book appearance in 'X-Men' #129, where Jean Grey becomes possessed by the Phoenix Force and transforms into Dark Phoenix.
The 'Dark Phoenix Saga' story arc was published in 'X-Men' issues #129–138, detailing Jean Grey's transformation into Dark Phoenix and the subsequent events.
The standalone film 'Dark Phoenix', starring Sophie Turner, was released, focusing entirely on the Dark Phoenix story arc and its impact on Jean Grey and the X-Men.