Krona is a fictional extraterrestrial villain in the DC Comics universe. Originally, he was a member of the ancient race of powerful, blue-skinned humanoid immortals from the planet Oa. He was a scientist obsessed with observing the creation of the universe, despite an Oan legend that said discovering that secret would cause a great calamity. Krona created a machine that allowed him to see into the moment of creation. Somehow, his experiment disrupted the process of creation, with terrible consequences. The exact consequences have been rewritten over the years; originally, it was the unleashing of evil itself in the universe; later, it was assumed to be the creation of the evil anti-matter universe of Qward. During the Crisis on Infinite Earths, it was also revealed to be the cause of the existence of all parallel universes in the DC multiverse; several villains were sent back in time to stop him, but were defeated by Krona and the other Oans. In a final revision, it was established that it increased entropy in the universe, shortening its existence by a billion years.
Whatever the exact effects, Krona's fate was the same: he was judged by his fellow Oans and sentenced to exist in an energy form, forever wandering the universe. The Oans, feeling guilty about not having stopped Krona, then argued how to deal with the results, eventually splitting into various groups (the Guardians of the Universe, Controllers, and Zamarons). However, Krona eventually escaped this fate and tried to start his experiment all over again, since the original machine had exploded before he could finish watching the universe being created. He was stopped by the Green Lanterns of Earth, Hal Jordan and Alan Scott, and banished into his energy form again.
Eventually, however, Krona's energy form somehow reached the dimension were the souls of the dead in the DC universe await their passage into their final residences. This realm was ruled by a being called Nekron . Because of the paradox of an immortal now being in the Realm of the Dead, a rift opened between the dimensions, which allowed Nekron to see the universe of the living. Desiring it, but being too large to pass through the gap, Nekron instead recreated Krona as an undead being of enormous power. Given an army of similarly-restored spritis, Nekron sent Krona to kill the Guardians, so the rift would open more. Krona accepted because part of Nekron's plan involved collapsing the universe so as to recreate it according to his own desires, which would give Krona the chance to see a universe being born. Krona and his minions suceeded in killing some Guardians as well as several Green Lanterns. However, he was defeated when Jordan entered the realm of the dead and incited the spirits of the recently-killed Lanterns to rebel against Nekron. This gave the Guardians enough time to banish Krona into the dead realm and close the rift.
In later appearances, Krona resurfaced as an agent of entropy itself (whether this contradicts the story in the paragraph above or not isn't clear.) One of his attacks caused the New Guardians to vanish.
Krona was also the main villain in the DC/Marvel JLA/Avengers mini-series. In it, apparently still an entropy-being, he destroyed several dimensions while trying to discover the secret of universal creation. He finally obtained it from the cosmic entity Galactus and tried to destroy both the Marvel and DC universes in order to create a new one, but the heroes managed to stop him, imprisoning him into a "cosmic egg" from which a universe will be born in about a trillion years. Since this story was part of a special publication by two separate comic book companies, it would not usually be considered canon, but this is a rare case in which it has become canon: the cosmic egg later reappeared in the pages of JLA.
