"I am become death, the destroyer of worlds."
There's been some chat recently about the lack of strong female villains in comics - characters that have the iconic status of Doctor Doom or The Joker. So what is Dark Phoenix? Chopped liver?
The Wikipedia list of supervillainesses (I'm not sure that's even a word. Surely "villain" is not gender specific) does not even mention her, and yet she has been one of the most influential and popular, not to mention one of the most physically powerful female villains in the history of comics. For many years after her first appearance, when a hero character went bad (particularly if female) they would be referred to as "Dark-" and everyone got the reference.
Okay, I know some might question whether Dark Phoenix counts as a villain because she is only a hero gone bad temporarily, but Catwoman is on the Villainess list and she's been swapping sides since the mid-1950's. And consider Dark Phoenix's thematic ancestor, Dr Jekyll and Mr. Hyde: can you see anyone ever arguing that Hyde doesn't count as a villain because he is only the dark aspect of Jekyll?
The storyline that first introduced Dark Phoenix is possibly the best thing Chris Claremont ever wrote. His work with John Byrne during this period produced some classic X-Men stories that were so effective and powerful that they are still getting recycled even now, and the slow build of the Dark Phoenix/Hellfire Club storyline was a masterwork of pacing. It's an excellent story on its own, but it reaches a whole other level when, in a classic case of snatching defeat from the jaws of victory Jean Grey triumphs over Mastermind's mind control by opening Pandora's box and letting out her dark side, which is kinda like beating someone at chess by blowing up the building where the game is being played.
In fact, in her original outing, Dark Phoenix only lasts two issues. Then she is shut down, and before she can escape again Jean Grey allows herself to be killed rather than become the monster again. But in that short time she eats a star - she may be solar powered but she is anything but environmentally friendly - and tears through a Shi'arr battlecruiser like a tank through butter. Sadly, we don't get to see her do much else as Claremont and Byrne realise that there is no credible way for the X-Men to defeat her unless they get tricksy, and so they backpedal a little and even though in Uncanny X-Men #135 Phoenix says she is leaving Earth forever, and her grasp of her powers is good enough that she is opening warp gates and consuming stars, she then feels the need for a little angst moment and pops home to mommy so they can have a rematch with her.
It's only because the pace is rattling along like the best action movies that your suspension of disbelief doesn't crumble at this point. Dark Phoenix is described as having Galactus level power, and that's before she consumed the star, and yet when she zaps Professor Xavier it just appears to squish his wheelchair and tear his clothes a bit. Okay, Prof X may be the most powerful telepath on the planet but Dark Phoenix snuffs out suns. When she says she is killing you I'd expect more than cuts and grazes on anything below god-level. They then go head to head in a telepathic battle and the Prof wins, of course. The rationale being that deep down, part of her was on his side.
But they couldn't leave it there. Like all great villains, Dark Phoenix was too good a character to throw away, even if she was dead. But the first sequel Claremont gave us was something different. Introducing Madeline Pryor as visually identical to Jean Grey, he plays with our expectations of what the connection might be between the two. The great thing about this story, which was utterly destroyed in later retconning, is that there is no connection. Madeline is just an ordinary person who looks like Jean. But in superhero comics where almost everyone the protagonist meets in the most mundane setting turns out to be a villain/demon/alien it was a masterly subversion of the reader and the characters' expectations to have the returning villain Mastermind play on these assumptions to make the X-Men think that this was Dark Phoenix resurrected, getting revenge on the X-Men by setting them up to murder the innocent, ordinary Maddy.
After this it goes downhill like a sledge on Jupiter and retcon is piled on top of retcon to the point where I kind of lose track. Jean Grey was never Phoenix in the first place and Maddy was some kind of demon clone or some such, and Rachel, Jean's daughter from an alternate future, inherits the phoenix power that Jean never had anyway and I lose the will to live.
Last I looked Jean was currently dead but nobody expected it to last, Dark Phoenix had a miniseries where she got to destroy the universe (or something) and she's about to get her big screen debut.