To do so, they created an energy source known as the Central Battery, an energy source comprised solely of willpower alone. The Guardians also created a device called a Power Ring, which could tap into the power of the Central Battery if the user had sufficient willpower. The wielders of these Power Rings were to champions of the universe and were to protect and help those in need. They were known as the Green Lantern Corps.
Eons ago, the Guardians imprisoned a being comprised of pure fear in the Central Battery. This beings goal was simple, to spread fear and in turn so destruction to feed off of that fear. The Guardians had no choice but to stop this entity known as Parallax. The color that embodied fear was the color yellow. Because of this, the Green Lanterns powers were ineffective against the color yellow.
The most noted of the Green Lanterns was Hal Jordan. Hal was the greatest of the Green Lanterns, but was also the one responsible with destroying the Guardians and the Green Lantern Corps as we know it. After draining the Central Batter to get enough power to bring back millions from the dead, Parallax entered Hal and they emerged as Parallax.
The Corp was decimated and with only one Guardian left, Ganthet, the Guardians appeared to be on the brink of extinction. Ganthet traveled to Earth and had to quickly find a replacement Green Lantern with the last known Power Ring. This fell upon Kyle Rayner, freelance artist, to do the job. John Stewart, an architect was later chosen as a backup for Kyle.
Parallax was ultimately defeated and returned to the Central Battery for a time, but broke out, threatening to destory all of the Green Lanterns. A resurected Hal Jordan faced off with the being that once possessed him. Parallax was defeated, and with his defeat, the color yellow can now be overcome by the will of the Power Ring user. With this over, Hal Jordan once again became Earth's Green Lantern.
The Guardians have since been restored and with the five remaining Green Lanterns, Kyle Rayner, Hal Jordan, Kilowog, Guy Gardner, and John Stewart, the Corps is alive again.