In the classic JLA/Avengers crossover from Marvel and DC Comics, Green Lantern once unlocked a powerful new form thanks to absorbing part of the Cosmic Cube. Kyle Rayner showed the ability to manipulate reality as the cosmic artifact recharged his Power Ring while offering him a new array of powers. But unfortunately, the mighty Green Lantern form is only accessible through the Marvel Universe, meaning the hero couldn't keep the power after the crossover.
The Justice League and Avengers clashed in the epic JLA/Avengers crossover from Kurt Busiek and George Perez, as the four-issue miniseries did the seemingly impossible and told an incredible, coherent crossover story between the DC Comics and Marvel Comics universes. The story featured the Justice League and Avengers racing to acquire several cosmic artifacts to save their respective universes while later fighting each other in some of the best superhero battles in comics. However, as the Green Lantern gained the power of the Cosmic Cube, things got even more complicated when the two universes were merged as one.
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In JLA/Avengers #2, Kyle Rayner's Green Lantern has his powers drained by Spectrum, leaving the hero weakened and unable to create constructs. But, when the hero sees the Cosmic Cube, he realizes he can get energy from the artifact. So he reaches out for its immense cosmic power, transforming him into a new form much more powerful than his usual Green Lantern self. Rayner notes that it packs a serious punch.
The shockwaves throughout reality would have significant impacts on Kyle Rayner and Hal Jordan, as the the veteran Green Lantern replaces the current guardian as time hitches.
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