JLA/Avengers (Issues 2 and 4 are titled Avengers/JLA) is a crossover limited series published by DC Comics and Marvel Comics, and features two teams of superheroes, DC Comics' Justice League of America and Marvel's Avengers.
In 1979, DC and Marvel agreed to co-publish a crossover series involving the two teams, to be written by Gerry Conway and drawn by George Pérez. Although work was begun on the series in 1981, editorial disputes prevented the story from being completed.
An agreement was eventually made between the two companies in 2002, with the story to be written by Kurt Busiek and drawn by George Pérez. In a joint panel at WonderCon 2000, Busiek, Avengers writer at the time, and Mark Waid, then-writer of the JLA, stated they had nearly come to an agreement to begin the crossover within the regular issues of their respective books but the two companies could not come to a business arrangement.
Avengers/JLA eventually became a 4-issue comic book mini-series jointly published by Marvel Comics and DC Comics in late 2003 through early 2004. The mini-series was published in the 48-page Prestige Format. Issues #1 and #3 were published by Marvel Comics under the title JLA/Avengers while issues #2 and #4 were published by DC Comics with the title Avengers/JLA. The story as a whole was reprinted by DC Comics in 2005 as a 2-volume collector's edition hardcover. It is considered canon.
Krona scans the Grandmaster's mind and learns of Galactus, while the Grandmaster uses the power of the twelve artifacts to merge the two universes and trap Krona at their center. Krona then retaliates by killing the Grandmaster and causing the universes to collapse. The Avengers and the Justice League join forces to battle Krona, and due to several chronal distortions are aided by past members of both teams. Krona is finally defeated, and implodes to become a "cosmic egg" from which a universe will eventually be born. The two universes are returned to normal with no evidence of Krona's tampering, and the cosmic egg is later revealed to be being monitored by the JLA.