What is the source of power for the Green Lantern?
Green Lantern Corps
The Green Lantern Corps ranks were decimated by the Parallax-possessed Jordan. The Corps' power source, the Central Power Battery, was drained by Jordan and destroyed, removing the original power source for the rings. One Guardian, Ganthet, survived Jordan's onslaught, and chose a young human artist, Kyle Rayner, as the sole Green Lantern of the Corps. Rayner would eventually become Parallax's nemesis, and during their first battle, Rayner intentionally triggered a chain reaction in Oa's core, causing the planet to explode.
Green Lantern/Superman: Legend of the Green Flame
Superman and Green Lantern encounter the power that killed them - the sentient Green Flame, the remains of the magic energies of Maltus. The Green Flame explains that their deaths were a result of Jordan trying to load his scientific ring with supernatural energies. Then it tempts Hal to give in for the supernatural power of the Green Flame instead. At that point, the Stranger appears, and teaches Hal how to tame the corrupt Flame. Hal reads the oath of Alan Scott, loads his ring, and the threat of the Green Flame is neutralized. The Stranger then returns Hal's and Superman's souls to their bodies, disposing of the lantern.