Parallax in the Comic Books
The history of Parallax in the comics is an extremely complicated one, so stick with me while I gingerly lay it out for you:
After the destruction of Hal Jordan’s hometown of Coast City by Mongul and the Cyborg Superman (following the death and resurrection of the real Superman), Hal was overcome with grief, survivor’s guilt, and a modicum of insanity, to say the least.
Hal decided that the only way to make things right again was to steal all the power of Oa's Battery – that gigantic lantern that sits at the center of Oa and powers the entirety of the Green Lantern Corps – so he could remake existence as he saw fit.
The smoldering corpse of Kilowog, the alien drill sergeant with a heart of gold
Hal fought and killed many a Green Lantern, including his dear friend Kilowog and his old enemy Sinestro (freed from Green Lantern prison by the Guardians in order to fight Hal), just so he could steal almost all the Battery’s juice and become Parallax.
I say he stole "almost all” of the Battery’s juice because there remained just enough for Ganthet – the last surviving Guardian – to reconstruct Jordan’s ring and give it to the Earthman/graphic artist Kyle Rayner, the last of the Green Lanterns at the time.
Some months later, Parallax reappeared and attempted to recreate the DCU in the major event known as Zero Hour: Crisis in Time, the ten year anniversary and less successful sequel to Crisis On Infinite Earths. All the remaining heroes of the DCU – Superman, Hal’s good friend Green Arrow, Waverider, Green Lantern (Kyle Rayner), Batgirl, and so on – fought and defeated Parallax, resulting in a slightly rebooted (but mostly the same) DC Universe. Again.
Parallax did not die, though – no sir! In fact, it wasn’t until another crossover event known as Final Night (not to be confused with Final Crisis) that he “died” sacrificing his life to reignite the sun and save the galaxy. What a helpful guy, that Parallax.
The deceased Hal Jordan then became The Spectre, otherwise known as “The Spirit of Vengeance,” ever-searching for redemption. Because nobody wanted to read a comic book about Hal Jordan as anything but a Green Lantern, the series was canceled just 26 issues in.
Enter 2004, Geoff Johns, and the wildly successful Green Lantern: Rebirth, where we learned that Parallax wasn’t just a cool name Hal Jordan came up with once he consumed the power of Oa’s Battery. Rather, Parallax was a super powerful space insect and the "Fear Entity" – the living embodiment of fear that represents the yellow energy that powers Sinestro’s ring – that had long ago been imprisoned inside the Battery. Parallax was the reason for the centuries-long “yellow impurity” that had rendered Green Lantern rings useless against anything yellow.
In Rebirth, we learned that Parallax influenced Hal Jordan’s emotions and actions for years and was the true cause of Hal’s megalomania and genocidal ways (not to mention his graying temples). Hal Jordan, of course, was resurrected, Kilowog was resurrected, the Guardians were resurrected, the Green Lantern Corps was reinstated, and Parallax was defeated, although he has returned from time to time, most prominently in the excellent 2006 crossover The Sinestro Corps Wars.
Parallax in the Movie
Parallax in the movie - besides being a gigantic, multi-tentacled space-cloud with a stereotypical grey alien head on top - is a strange combination of Green Lantern villain Krona, the retconned version of Parallax from the comics, and that giant sandstorm from 1999's The Mummy.
Krona + Comic Book Parallax + The Mummy sandstorm = Movie Parallax
I won’t go into the long, complicated, and repeatedly retconned history of Krona (you’re welcome); suffice it to say, the most recent version states that Krona is a rogue Guardian who wanted to utilize the other colors of the emotional spectrum – yellow, red, blue, violet, rainbow, and indigo – to more effectively control the universe. The rest of the Guardians obviously didn't go for that.
Similarly, Parallax from the movie was also a rogue guardian. In this case, he entered the yellow chamber of fear energy long ago – not for evil purposes, mind you, but because he felt that the fear energy would allow the Guardians to better police the universe – and inadvertently became the aforementioned many-tentacled space cloud with a head. You can’t make this stuff up, people.
In case you were wondering, this is how you suck out a "fear skeleton"
Abin Sur – the Green Lantern whose ring would eventually go to Hal Jordan – fought and imprisoned Parallax in "The Lost Sector." Of course, Parallax escapes some years later and flies around the universe sucking out victims’ “fear skeletons,” which kills them immediately. This is the conflict at the heart of the Green Lantern film: Parallax is coming for you and he’s going to suck out your fear skeleton. (Yawn.) I say again, people, you cannot make this stuff up. Well, I mean, technically the writers of Green Lantern did make this stuff up, but what I mean to say is – they probably shouldn’t have.
Advantage: The Comic Books – because as needlessly complicated and retconned as they are, nothing’s worse than a massive yellow space cloud that sucks out your "fear skeleton."