Nekron is a fictional embodiment of Death and ruler of a region adjoining Hell that also seems to border on Limbo and Purgatory within the DC Universe. It is where the souls of the dead await passage to their final residence in either the Silver City or Hell. Nekron draws his power from the souls and spirits of all those who have ever died.
After the Oan scientist Krona was sentenced to banishment as pure energy, that energy somehow reached Nekron's realm; a rift opens between the dimensions due to the paradox of an immortal now being in the Realm of the Dead. Desiring the living world he is now able to see, but being too large to pass through the rift, Nekron recreates Krona as an undead being of enormous power instead. Given an army of similarly-restored spirits, Nekron sends Krona to kill the Guardians of the Universe in order to increase the size of the rift. Krona cooperates because part of Nekron's plan involves collapsing the universe so as to recreate it according to his own desires (which would give Krona the chance to see a universe being born). Krona and his minions kill several Guardians and Green Lanterns while destroying the Central Power Battery to prevent the Lanterns from recharging their power rings. Although his attack is powerful enough to shatter the morale of the Green Lantern Corps, Hal Jordan manages to inspire and rally his fellow Lanterns into attacking Krona with the charge still left in their rings. Nekron is defeated when Jordan enters the realm of the dead and incites the spirits of the recently-killed Lanterns to rebel against him. This distraction undercuts Krona's power supplied by the being and thus gives the Guardians enough time to banish Krona into the dead realm and close the rift with Jordan still trapped inside. The spirit of Jordan's predecessor, Abin Sur, helps him escape the realm.
Captain Atom has a run-in with Nekron in which he is described as "Death as the Ultimate Opponent". Black Racer, also making an appearance, is coined "Death as an Inevitability", while Death (prominently known from her appearances in Vertigo titles) represents "Death as the Release, as Mercy, as Compassion." Drawing into the Quantum field, Captain Atom is able to get the better of Nekron and is able to visit his deceased wife.
Nekron later battles the demon Nebiros, following the destruction of Doctor Fate's Tower Of Fate. Also appearing in this issue is Jared Stevens (Fate).
Kyle Rayner later encounters Nekron when he nearly reenters the living universe when the level of supernatural activity mysteriously increases. This increase gave Nekron the opportunity to briefly control every deceased member of the Green Lantern Corps, which act as both his army and his anchor to the living universe. Kyle was able to push Nekron back into his dimension by freeing the undead Lanterns from Nekron's control. The rift is eventually sealed following Nekron's defeat.
Nekron also encounters Wonder Woman and Rebecca Carstairs (Witchfire).
In the "Blackest Night" storyline, Nekron is revealed to be responsible for the creation of the black power rings, which reanimate the dead, and is collaborating with Scar and Black Hand. He's also responsible for the imprisonment of the Anti-Monitor during the aftermath of the Sinestro Corps War on the planet Ryut. When the Black Lanterns' power levels reach one hundred percent, he rises from the Earth, just outside of Coast City. Nekron is also revealed to be the true mastermind behind the miraculous resurrections of several superheroes in the past (a deed previously thought to be a consequence of Brother Blood tampering with the forces of life and death). By "allowing" the dead heroes to be reborn he grants himself a small cadre of "inside agents"; by briefly reanimating Batman's clone (created by Darkseid during Final Crisis and implied that the dark lord's awareness of the remains is not authentic) as an emotional tether to stimulate a powerful enough emotional response, Nekron is able to create black rings that latch on to Superman, Wonder Woman, Donna Troy, Kid Flash, Animal Man, Ice, Green Arrow and Superboy, transforming them all into Black Lanterns; Hal Jordan and Barry Allen are similarly targeted, but evade sharing their comrades' fates thanks to Barry taking himself and Hal two seconds into the future and causing their would-be rings to shut down.
Nekron begins to reveal the full extent of his plan. Nekron and his Black Lanterns bring the subdued Guardians to a clearing where the same symbol that was seen on Professor Zoom's chest is drawn in what appears to be blood. After questioning one of the Guardians and getting him to admit that he can no longer remember why the Guardians took a vow to protect the universe, Nekron slits his throat and Black Hand removes his organs and places them in the center of the symbol. After crushing the organs with his scythe, Nekron calls forth The Entity. The Entity is the white light given physical form. When Nekron strikes the Entity every living thing in the universe cries out in pain, this is because the Entity is what gave birth to life and all living things came from it. Nekron intends to kill the Entity and thereby kill every living thing in the universe. His plans, however, are thwarted when Sinestro bonds with the Entity becoming a White Lantern.
In the battle that follows, the Life Entity eventually abandons Sinestro, refusing to be controlled by his ego, and bonds itself to Hal Jordan, who states that Nekron merely opened the doors between life and death while it was still the heroes' choices to return to life. Hal turns the power of The Entity on Black Hand (Deadman having discovered that Black Hand functions as Nekron's link to the living world), causing Black Hand to return to life and vomiting one white power ring that enters the Black Power Battery and resurrects the Anti-Monitor (destroying the Power Battery in the process), who immediately attacks Nekron. Nekron then returns the Anti-Monitor to the Anti-Matter universe. Twelve other white rings are then released by Black Hand and seek out several deceased heroes and villains who had been recruited into the Black Lantern Corps and turns them into White Lanterns instead. In the final confrontation that follows, Nekron is banished yet again, with Hal Jordan and Barry Allen speculating that death will now be more permanent with Nekron's plan having failed.
At the conclusion of the miniseries Brightest Day, Swamp Thing appears, grown to towering size and emerging from the forest at the former heart of Star City, which has become corrupted. The White Lantern Entity explains that just as the memories of Alec Holland acted as a template for the Earth Elemental, the personality of Nekron has been imprinted over it. The corrupted Swamp Thing does battle with Alec Holland (having been resurrected and turned into the new Swamp Thing by the White Lantern Entity), who is aided by embodiments of the four elements (Hawkman, Hawkwoman, Firestorm, Aquaman and the Martian Manhunter) and is defeated, driving out the influence of Nekron once again.
The limitations of Nekron's powers are unknown. Geoff Johns has described him as the most powerful dark force in the DC Universe. He has displayed the ability to raise the dead, kill (even Guardians) with a touch, fire bolts of black lightning and grow without limit. He is also capable of fighting against universal forces like the Life Entity, who was bonded with Sinestro. Nekron also has incredible durability and reality warping powers, as he was able to withstand a blast from the Anti-Monitor and send him back to the anti-matter universe.
In the alternate timeline of the Flashpoint storyline, it is revealed that Nekron was unleashed into the Universe when Atrocitus killed William Hand. Where Nekron and his Black Lanterns waged war is left unknown. All that is known is that the Guardians of the Universe tasked Abin Sur the mission to travel to Earth and recover the White Entity and bring it to Oa.