Nekron is a cosmic supervillain and dark god originating in the DC universe and is mostly an enemy of the Green Lantern and Captain Atom though his grand-scale could very well put him in league with any of DC's big villains if he ever succeeded in his goals. It is confirmed by DC that Nekron is not, as was thought, the personification of Death, but in fact the embodiment of "Black', the little-understood conceptual energy that opposed the light of creation, and once claimed the living universe before being driven off by the being that introduced the White Light of creation (left ambiguous, but implied to be what people traditionally refer to as "God" or "the singularity", or other such reasonings for the creation of life)
Nekron is the ruler of a mysterious dimension known as the Land of the Unliving, where the souls of people who have died pass through in their search for either Heaven or Hell, this makes Nekron in some ways similiar to a traditional representation of death itself - though he is not so, though he has been confirmed as a being that may embody the darker aspects of mortality. However the dark god's life (or lack of) would be changed forever when Krona entered his realm, the paradox of an immortal being in the realm of death tore open the fabric of space and thus allowed Nekron to observe the living universe that lay beyond his own shadowy realm. Feeling imprisoned in his own realm (which, in fairness, he seems to be) he sought out many ways to escape the confines of his world so as to enter the living universe.. Thankfully for the inhabitants of DC he has yet to do so - though not for lack of trying.
It is revealed that Nekron is the leader of the Black Lantern Corps during the Blackest Night, the one who imprisoned the Anti-Monitor on Ruyt and collaborating with the former Guardian of the Universe Scar on eradicating the universe of all life. It was then revealed that he was the one responsible for the mysterious resurrections of past heroes, thus allowing them them to be Black Lanterns. He was defeated after Hal Jordan bonded with the White Light of Creation, bringing Nekron's link to the living world, Black Hand, back to life.
Nekron is depicted as a being of immense power, possibly limitless in scale - though he seems unable to enter the living universe. Among the least of his known abilities are raising the dead, firing bolts of dark energy, killing with a single touch and growing without limit.