The Black Lantern Corps is a Lantern corps powered by the emotional power of darkness/nothingness. Effectively compromised of the reanimated bodies of the dead and created by Nekron, the corps' goal is the consume all living things in the universe and destroy the white light spectrum of creation. The emergence of the corps had beeen long prophecised as "The Blackest Night", by the Guardians of the Universe.
Similarly parasitic like red, orange, sapphire and indigo rings, the black lantern rings actively seek out and reanimate the dead that would cause the most emotional trauma to their living targets, as this would cause their victims' hearts to be as full of emotions as possible when they are consumed. The greater amount and variety of emotions consumed by a Black Lantern, the more energy is provided to power the Black Lantern power battery.
The Darkness was in existence before the creation of the universe. Banished at the dawn of time by the white light of creation, its constant attempts to return caused the white light to be fractured into the emotional spectrum. When the Guardians of the Universe first harnessed the Emotional spectrum, Nekron was formed out of the darkness as a defense mechanism to become the Guardian of Darkness. Given shape in the form of life's idea of death, Nekron was driven by an unending hunger to consume all life and thusly restore the darkness to power. In his attempt he was eventually stopped by the Guardians.
Intending to use the same tactics as the Guardians, Nekron created the Black Lantern Corps to bring about his arrival onto Earth to call forth The Entity; the white light given physical form, and kill it, thereby killing every living thing in the universe.
To enact his plans he began by allowing select heroes to escape the realm of death by leaving it open for them to escape. By "allowing" the dead heroes to be reborn he grants himself a small cadre of "inside agents" which he could turn when required. These heroes included Superman (Kal-el), Wonder Woman (Diana Prince), Donna Troy, Kid Flash (Bart Allen), Animal Man, Ice, Green Arrow (Oliver Queen), Superboy (Kon-el), Green Lantern (Hal Jordan) and Flash (Barry Allen).
Furthermore, following the emergence of the Sinestro Corps War, Nekron exposed his power to a Guardian who had been burned by the Anti-Moniter which had poisoned her and killed her slowly. Given life by Nekron, the Scarred Guardian (nicknamed "Scar") made moves to further the War of Light which was prophecised to lead to the Blackest Night, she also orchestrated the creation of a power battery to be built around the corpse of the Anti-Monitor and placed in the dead space sector 666 to serve as a power source for the Black Lantern Corps and allow Nekron to arise in within this universe.
Scar then seeks out William Hand who was to be the physical embodiment of death for the corps much like Ion was the embodiment of will or Paralax for fear. Ressurrecting him as the first ring-bearing member of the corps, Hand then proceeded to use the skull of the clone of Bruce Wayne to unleash the Black Lantern rings on Earth.
While Nekron's plan reaches fruition it is ultimately foiled when The Entity begins ressurecting the Black Lanterns and his tethers to the mortal plane (William Hand and the Anti-Monitor). Thusly ending the Blackest Night, destroying the Black Lantern Corps and banishing Nekron back into his realm.
As an avatar of death, Nekron is seen as "Death as the Ultimate Opponent". The Black Racer, another avatar of death, is coined as "Death as an Inevitability", while Death herself represents "Death as the Release, as Mercy, as Compassion."
By briefly reanimating Batman's clone as an emotional tether to stimulate a emotional response, Nekron transformed his "inside agents" into Black Lanterns although they are all eventually freed from his thrall through various means. Diana herself was revived by the memory of and love for Bruce.