The Necromancer is the first of the big players to arise in the wake of the Azure Flames. Like all of the others, he is a consequence of the pre-cataclysm conspiracies to establish a global tyranny. Unlike them, he is a consequence in the most literal sense: he had no ties to either of the conspiracies, and instead arose because of the effects of their failure.
The Necromancer was a ghetto kid, son of a waste of a mother and abandoned before birth by his father, and kept in check only so much as it kept his mother in the good graces of the authorities. He got shot when a firefight between street gangs broke out over a particular corner of the drug trade, and the gang-bangers (being notoriously incompetent shooters) cared not where their fire went. As he got rushed to the hospital, the cataclysm began; he was abandoned at the operating table, dismissed as a worthless punk kid better off dead, and left to die.
As he died, Christopher Walken appeared to him. Only it wasn't Walken, but someone appearing as Walken did in The Prophecy, calling himself an angel of God and offering the boy a chance for revenge- to make the world feel his pain, listen to his word, and obey his commands. The boy agreed, and the angel--who is Satan--gave the boy over to Legion.
Legion is the source of The Necromancer's power. He does not control the boy, as the boy is not dead and Satan forbade Legion controlling the living. Legion abides because his desires are being fulfilled, as he now controls billions of corpses, but chafes at being subject to a boy's borrowed authority (as he serves as Satan's anchor on Earth). Satan is the deniable Grand Vizier to The Necromancer, playing the boy like a fiddle as he knows the boy's psychology and pushes his buttons as a master pianist plays the keys.
The Necromancer has other henchmen at his disposal, which are the damned souls of the worst of Mankind allowed to take up the dead flesh at The Necromancer's disposal and walk the Earth once more to fulfill The Necromancer's will. Other damned souls are yoked to serve as immaterial shades, advising The Necromancer. All of these are withdrawn once Satan removes his support, albeit not at once, and their removal serves to track progress in the war against The Necromancer; until that support is withdrawn, they return time and again to menace the enemies of The Necromancer.
The Necromancer, billed as "Master of All Flesh", endures for as long as he does because he and Legion cooperate. They erect a worldwide Empire of the Dead, complete with ziggurats and sacrifices, following Satan's advice. However, Satan (being the Supreme Deceiver) ultimately betrays both his human and his demonic ally once their usefulness is at an end and he shifts his allegiance to the Empire of Man. Knowing his allies' weaknesses, Satan elevates the Empire and enables their conquest of The Necromancer; providing verifiable proof of The Necromancer's actions drives the Empire of Man's propaganda efforts that galvanize the people to support the Emperor. The Necromancer ends his life as it began: mortally wounded, on a table, and abandoned to die. The Emperor, at the final moment, recognizes that his enemy is truly at his end and gives him the mercy of a swift, painless death. The Necromancer then goes to Hell.
The final death of The Necromancer marks the end of the first phase of the world post-cataclysm, and the shift from surviving in a hostile ruined world to the rise into a recovered world filled with terrible purpose and horrific fury at that which ruined what came before.