In the vast X-Men universe, there are very few stories as revered as "The Dark Phoenix Saga"; even more so the creative team behind it. Written by Chris Claremont with art by Dave Cockrum and John Byrne, "The Dark Phoenix Saga" is one of the greatest examples of complex comic book storytelling ever created. However, it may be prudent to break down the saga for those unfamiliar with the multifaceted storyline; any X-Men saga has a tendency to be very, very complicated.
If you're looking for "The Dark Phoenix Saga" explained, the first step is to understand th comic book version is actually broken into two parts: the "Phoenix Saga" and the "Dark Phoenix Saga." Although it is too early to say exactly how the film will encompass both parts, the former is essentially the rise of the Phoenix power and the latter is the fall.
The Phoenix Force is a tad complex to explain, but it is essentially an ancient, sentient being that manifests change. It can destroy and create universes by sheer will—it "burns away" what is stagnant and grows new life from the ashes.
It has traveled the universe—sometimes as formless energy and other times taking a host—for billions of years. While hostless, it usually goes by the name "Phoenix Force"; when occupying a host, it usually calls itself "Phoenix."
When Jean Grey was young, her vast mutant abilities were noticed by the Phoenix Force and it consciously decided to watch her development.
At the top of Uncanny X-Men #101, Jean was exposed to a deadly amount of solar radiation as she and the rest of the X-Men escaped a space station run by the staunch anti-mutant bigot Stephen Lang. As she was slowly dying from the poisoning, she called out to the Phoenix Force and it answered.
Jean's only wish was to save her fellow X-Men and, in response, the Phoenix Force took her form, memories, personality and some of her telekinetic powers. In this new body, the newly dubbed Phoenix was able to save the rest of the X-Men; however, it placed the original Jean into a coma-like state at the bottom of Jamaica Bay where she could heal from the damage.
After taking Jean's place on the X-Men without their knowledge, the Phoenix was able blend in without a second look.
On a distant planet lay the M'Kraan Cystal; a large, pink jewel that housed, among other things, the point where all matter and antimatter in existence meet. In the "Phoenix Saga," Emperor D'Ken of the Shi'ar Empire decided to harness this power for his own sinister purposes.
His sister, Lilandra, knew this would destroy the universe, so she summoned the X-Men to aid her in stopping D’Ken. However, during the process of harnessing this power, the crystal was damaged and the fate of literally all existence was on the line. Using its immeasurable power, the Phoenix was able to repair the damage and save reality from certain doom.
The Phoenix continued with the X-Men, battling various villains and experiencing human emotions for the first time. It was these experiences that started to break down the Phoenix’s mental state, allowing the evil mutant Mastermind to infiltrate its mind and wreak havoc on its conscience—a conscience already questioning human morals as only an all-powerful being could.
This warping of sanity and morals by Mastermind leads to the "Dark Phoenix Saga."