The Joker is chaos incarnate, a man so insane that his own motivations are rarely understood, even by Batman. Meanwhile, the Spectre is DC's spirit of vengeance and the physical embodiment of divine wrath. Despite its power, the Spectre must have a human host to keep it in check otherwise its thirst for vengeance would punish everyone regardless of the severity of their sins. While these two elemental figures couldn't seem further apart, the ultimate agent of chaos merged with the most powerful being in the DC Universe in John Ostrander’s and Tom Mandrake’s The Spectre #51.
When a nightclub called The Killing Joke has opened up in New York City, rich socialites gather in fancy clown costumes to celebrate the Clown Prince of Crime himself. They are obsessed with keeping up with the latest trends and supervillains, and the Joker is all the rage in the news. They even extend an invitation to the Joker, who naturally accepts because as far as he is concerned the more smiles the better off the world will be, out of his own twisted vanity.
Batman tracks the Joker down to this nightclub and is about to apprehend his archnemesis when the Spectre appears. The Spectre declares that the Joker is one of the most vile people on the planet and is deserving of the harshest punishment available before his soul is finally released to hell. Despite his at times brutal methods, Batman does not believe that killing equates to justice. So the very human Batman attempts to bargain for Joker’s life with the gargantuan embodiment of holy wrath, He states that the Joker does not understand good from evil, that he is without a conscience and therefore should be considered innocent. The Spectre does not kill innocents, so he agrees to search the Joker’s soul to see if Batman’s hypothesis is correct.
However, a few issues earlier, the Spectre had been stabbed with the Spear of Destiny, which significantly lessened his powers at the time. In the Spectre's weakened state, the Joker’s fractured mental state and tainted soul overwhelm the Spectre. Jim Corrigan, the human host of the Spectre force, finds himself locked inside the soul of the Joker, while the Joker has fused with the Spectre in the material plane.
A madman with godlike powers is a dangerous thing, but luckily the Joker’s own personal vendettas prevent him from doing anything too drastic. Even though he now possesses the power to wipe out all life on Earth or pretty much do anything else he wants, he decides to torture the one man that mocks him by his very existence, Batman. He toys with Batman, summoning monster cars, cartoon mallets, and the like in order to kill his greatest foe.
Meanwhile, inside the Joker’s soul, Jim attempts to discover the secrets of the Joker and get him loose from the Spectre Force. Within Joker’s soul, Jim finds that he indeed has no conscience, in place of it his humor, pleasure, and desire have been overloaded and compose most of his being. Jim summons what powers he can from within Joker’s soul and briefly reignites Joker’s long-dead conscience.
This sends the Joker into a fit of hysteria, where he feels the guilt of all the people he has murdered. The Spectre Force reverts back to Jim, and Joker recedes inside his mind, entering a catatonic state. When Joker awakens, he will have no memory of having this power or even ever so briefly having a conscience.
Despite all of the Joker's maniacal plans, The Spectre #51 shows that he does not have the forethought to truly become more than Batman’s greatest foe. When given god-like powers from the Spectre, he wastes them on parlor tricks that could torment Batman instead of getting the real last laugh.