In the 60's, the Green Lantern concept was expanded from its original "mystical ring" role. Instead of being one guy, it became a legion of superheroes, each wearing a green ring and protecting a certain sector of space from invaders. Their bosses, who issued the rings and controlled their power source, were small, wizened aliens "the Guardians" whose tiny bodies housed great power.
Sound a little like Star Wars? Well, both Green Lantern and Star Wars borrow heavily from an old scifi book called "Lensmen." It was a great concept for a comic book, especially with the power rings. An imaginitive Lantern could conjure up anything with his ring (tanks, monsters, giant hamburgers). The ring had to be recharged every 24 hours, so running out of power was always part of the drama.
This idea was pretty good, but Green Lantern never caught on much with fans. Hal Jordan, who was the main Green Lantern throughout most of the run, was bumbled by writer after writer. The ring was amazing, and the cast of villains looked cool, but Hal never had much of a personality. His supporting cast (including an Eskimo named 'Pieface') was bland at best.
After decades of low sales, series cancellations, and fan indifference (besides a small, rabid few), DC Comics was ready to give free reign to Marz in "Emerald Twilight". He solidified Jordan's personality, which had been everything from "generic defender of justice" (early days) to "fickle, midlife crisis hero" (early 90s). Marz made Jordan into the misused servant of the Guardians, a talented and driven man who had been demoralized by years of service to these self-righteous 'tyrants'.
After his hometown is destroyed while he is away fulfilling an insignificant mission for the Guardians, Jordan is overwhelmed with grief. When rebuked by the Guardians, his anger and pride emerge, and he decides to seize the source of Green Lantern power and use it for his own purposes. In a Darth Vader-like move, the most skilled Green Lantern in history destroys the whole group.
Very interesting stuff, and what Jordan does after seizing power (Zero Hour and Final Night) is good too. People balk at making Jordan a 'villain', but let's face it, until Emerald Twilight, he was a superhero has-been at best.