Nekron is the ruler of a dimension that borders Hell, Limbo and Purgatory. His realm serves as a place where the departed souls of all who have died await passage to their final destination, good or bad. Since Nekron receives power from the souls who inhabit his realm, he is constantly trying to obtain more.
When the rogue Maltusianscientist Krona is banished by his brethren and trapped in energy form, he somehow reaches Nekron's realm. This results in a paradox that Nekron uses to his benefit. The paradox is caused by an immortal being like Krona occupying a realm meant for the dead, which results in Krona’s immortal energies leaving a tear in the time and space between the mortal universe and Nekron's realm. The tear allows Nekron to see into the universe of the living, but is not big enough for a being as powerful as Nekron to pass through. Wanting this universe as his own, Nekron recreates Krona as a being of immense power and sends him along with an army of similarly empowered spirits to kill the Guardians of the Universe so their immortal energies will tear the rift between the two realms wider and allow Nekron to travel between the two.
Krona and his new army are able to murder some of the Guardians, along with some of the Green Lantern Corps during their first attempts, and they are also able to destroy the Central Power Battery of Oa, preventing any of the surviving Lanterns from recharging their rings. Hal Jordan, in his usual attitude of never giving up, rallies the remaining Green Lanterns together, and convinces them that they should at least use everything their rings have left to try and stop Krona from advancing any further. Krona is eventually defeated when Hal is able to cross over into Nekron's realm and convince the souls of the Green Lanterns that had died in the recent attack to rebel against Nekron. This distraction gives the remaining Guardians enough time to banish Krona back into Nekron's realm of death and close the rift between the two dimensions.

Conflicts with the Living

Nekron later faces the hero Captain Atom in an attempt to use the Quantum Field as a means to pull the living, mortal realm into his dimension, thus expanding Limbo, the Land of the Unliving. Captain Atom is able to thwart this attempt.
When the Tower of Fate in Salem, Massachusetts is destroyed, Nekron battles the demon Nebiros for control over the mortal plane, but neither achieves his goal.
Nekron then imbues power rings with the 'power of death' and raises the corpses of the Green Lantern Corps. He is almost able to reopen the rift, but is stopped by Green Lantern Kyle Rayner.
At a concert in Boston, Massachusetts, the neophyte sorceress and celebrity Witchfiremistakenly uses a powerful spell which unwittingly summons Nekron to Earth. With the aid of Wonder Woman, Witchfire is able to push Nekron back into Limbo and re-seal the rift she had opened.

Blackest Night

Nekron Rising
Nekron Rising
Nekron is revealed to be the one behind the events of the Blackest Night and the trapping of the Anti-Monitor in the Black Lanterns' Power Battery and is the leader of the Black Lantern Corps. Nekron's goal in the Blackest Night is to restore the universe to a quiet, perfect order by eliminating the chaos wrought by the emotional spectrum. In doing so, he threatens to expose the Guardians' greatest secret. That secret being that the White Light Entity, which triggered existence itself, originated and is kept on Earth and the Guardians had long sought to keep its location a secret, even avoiding making anyone from Earth a Green Lantern until recently so as not to draw attention to the planet.
Nekron ultimately makes contact with the Entity by slaying one of the Guardians of the Universe and attempting to smite it with his scythe. Sinestro eventually bonds with the Entity and battles Nekron for the fate of existence. Sinestro fights valiantly, but is ultimately forcibly separated from the Entity by Nekron only for Hal Jordan to take control of it in his stead and used its power to resurrect those who had been turned into Black Lanterns while still living. Hal then relinquishes its power to resurrect Black Hand as well, thus compromising Nekron’s connection to the living world. The Entity seemingly possesses a mind of its own as it restores life to the Anti-Monitor as well, further damaging Nekron’s power source before ultimately banishing Nekron back to his realm by tearing him apart with White Lantern Rings, white being the combined power of the entire light spectrum.

Origin of Nekron

During the Blackest Night, it is revealed through Black Lantern Jean Loring that Nekron is the universe's response to the light of the emotional spectrum having pierced the darkness of existence. The Black Lantern claims the Guardians of the Universe's mission is to protect the light, not the universe, for "the universe was never theirs to protect." Eons after the coming of sentient life, the darkness which is the universe created its own guardian. A being which took on the appearance that life had of death. That Guardian is Nekron.

Alternate Universes

Flashpoint
Nekron is unleashed from imprisonment when William Hand is murdered by Atrocitus. He then declares war on all life in the universe. With the nigh-unstoppable Black Lantern Corps at his side, Nekron begins moving from one sector to the next, systemically wiping out every trace of life in each one that hasn't already been obliterated by the roving armies of soulless, renegade Manhunter drones. The Green Lantern known as Kilowog dies trying to stop Nekron and his forces from destroying Sector 674.
Star Trek/Green Lantern: The Spectrum War
In an alternate reality Nekron eventually escaped his imprisonment from the Dead Zone and destroyed all life in the universe, save for the Guardian of the Universe called Ganthet, who died conducting a ritual called the Last Light, which teleported a small handful of Lanterns and the husk of the living world called Mogo into another timeline. Unfortunately, Nekron was able to follow the survivors of his onslaught to their new home, happened upon the remnants of a dead world called Vulcan and commanded its inhabitants to rise from the ashes as Black Lanterns.

Powers and Abilities

Dark Lightning: He has the ability to fire bolts of a force called "dark lightning" from his hands, powerful enough to hurt and even kill the Guardians of the Universe. Despite being immensely powerful, it is not unstoppable. Wonder Woman was able to block it with her bracelets, although the force still threw her across the room.
Necrotic Empowerment: Nekron gains physical strength from the deaths he causes.
Reanimation: As the Lord of the Unliving, Nekron can reanimate corpses into fully functioning zombies working entirely under his control, and retaining semblance of their past personality.
Fatal Touch: The sustained touch of Nekron's hand can chill any mortal being to death, through supernatural means.
Immortality: Since he is not alive in the sense that we understand it, Nekron is technically incapable of being killed. As such, defeating him is extremely difficult; if defeated he will simply return to the Land of the Unliving.
Reality Manipulation: Nekron can manipulate reality on a planetary scale.

Weaknesses

Link Required: For all his power, Nekron holds one great weakness: he cannot exist in this universe. In order to properly interact with the living, Nekron requires a link between between New Earth and the Land of the Unliving, such as a tear in space-time, or a mortal link such as Black Hand.