Superman brings back to Earth an ancient Kryptonian device of unimaginable power called the Eradicator. After discovering that the device is attempting to recreate Krypton on Earth, Superman inactivates it.
Without his knowledge, the Eradicator affects Superman's personality, morphing him into a perfect Kryptonian being who is completely rational but who also despises anything associated with emotion. These new attitudes define Superman's, and Clark Kent's, reactions to those around him, including his ability to fight off alien assassins.
Superman must realise that Krypton's legacy is not his own, and that the wishes of a dead planet cannot define his future.
After killing three Kyrptonian criminals, Superman is tormented by feelings of extreme guilt. He makes the decision to undergo a self-imposed exile in space, where he is forced to fight in gladiator games and where he encounters a Cleric who gives him a Kryptonian artefact called the Eradicator (see story arc Superman: Exile). Superman's action attract notice of others, including Maxima who sends her handmaiden Sazu to evaluate Superman's potential (See Action Comics #645).
Soon after Superman returns to Earth, Jimmy Olsen begins to undergo an uncontrollable change, becoming similar to plastic man. Professor Hamilton discovers that exposure of viruses Superman brought back from space to the Eradicator are responsible for the transformation, and Superman decides that for the time being the safest recourse is to bury the artefact at the South Pole. While there, the Eradicator begins building an enormous structure, and initiates plans to transform Earth into a new Krypton. After struggling to stop the Eradicator's machinations, Superman finally inactivates it by undergoing a Kryptonian ritual in an induced dream state (see The Adventures of Superman #459-461).
Undergoing the Kryptonian maturation ritual affected Superman more than he could have realised. He begins to see that rationality and emotion are opposing forces, that to be civilized means to overcome and destroy - without passion - everything that is primitive. Although he couldn't realise it, Superman's attitudes are becoming more traditionally Kryptonian, emotionless and logical; the persona of Clark Kent begins to fade into the background.
Superman's actions on Warworld have ramifications that Superman could not have foreseen. One of the warriors Superman fought, Draaga, is seeking Earth. Draaga desires a rematch, and an opportunity to regain his honour. In direct competition with Draaga is the alien Lobo, who is also heading to Earth to kill Superman and win a bet. And thirdly there is Maxima who has heard of Superman's extra-solar exploits and who wants Superman for herself, for a mate.
Ironically, the same changes that are destroying Superman's identity are also saving him, and it seems unlikely that Superman could have defeated all of the incoming foes without the changes imparted by the Eradicator. The Eradicator allows Superman to invoke his mental powers to activate some of the Kryptonian machinery left behind in the Arctic and defeat Lobo. Similarly, the Eradicator's changes allow Superman to overcome Maxima's telepathic simulation.
These changes are paralleled by others: Superman is now identifying himself as a Kryptonian more than an Earthling, he garbs himself in Kryptonian costume, and he believes that Kryptonian evolution and culture are superior to all others; as a superior rational being, he must be mindful of the primitive nature of those that surround him.
It seems as if the soul searching Superman underwent while in exile was for naught: when Draaga requests to die in battle, with honour, Superman is not repulsed - it is logical to allow this primitive this wish. The only thing that prevents Superman from killing once more is an opportune intervention by Hamilton, who uses his teleporter.
Even the Kents - Jonathan and Martha - can't get through to Superman, who now refers to himself only as Kal-El. It becomes clear that Superman has been affected by the Eradicator, and that he did not truly succeed in suppressing it previously. It is only when the Eradicator begins to disregard Superman's wishes, especially in regards to preserving the welfare of the Kents, that Superman realises he must struggle against it. Finally, with the help of Pa Kent, Superman is able to pacify the Eradicator.
Once more Superman finds himself in space carrying the Eradicator, but this time it is to destroy it forever. Superman casts the device into the sun, knowing that at the same time he is destroying another vestige of his heritage.
Superman returns to Metropolis ready to resume his life, but he still needs to deal with the damage he's responsible for in the time since his return from exile. In particular, under the influence of the Eradicator Clark has been rude to the people who surround him, but aside from his parents, none of them know the reason why.
Also, Eradicator has proved itself as not merely a dangerous force, but a malicious one too. Although Superman assumes that he's been able to destroy the Eradicator, he is mistaken (see Superman: the Man of Steel #1).
Effect on Superman mythology
This arc, and the events leading up to it, introduce for the first time (in the post-Crisis timeline) Superman's base in the Antarctic, although it doesn't have an explicit name yet. The story also adds more to reader's knowledge of Krypton's history, including role of Superman's direct ancestors in building the Eradicator and thus being integrally involved in the bloodshed on that planet.
The changes undergone by Superman in these stories strongly define the character of Krypton, and suggest that Superman's role as a superhero is necessarily one of self-definition. That is, rather than Superman arriving on Earth as a superhero, this arc shows that he arrived on Earth merely as "super", and that the choice to become a "hero" was not a necessary one.