Kryptonite is a glowing green radioactive mineral from the planet Krypton, that was created as an aftermath of the terraforming of the World Engine and the Black Zero in the Indian Ocean and Metropolis. When Superman and the army destroyed the two, the numerous pieces of Kryptonite were founded as a result. The mineral's properties make it the greatest weakness of all Kryptonians, as its radiation is extremely toxic, potentially deadly to them. 
Prior to the World Engine's destruction over Indian Ocean by Superman, it managed to terraform the small portion of submerged land under itself into large nugget of emerald-green Kryptonite. The Black Zero accomplished a similar feat prior to it's destruction, as during the city's restoration, Lex Luthor discovered a small sliver of the same xenomineral, and promptly had it brought into close proximity to General Zod's corpse, so as to test it's side-effects on Kryptonians. Satisfied on finally discovering an exploitable physical weakness of Superman, Lex promptly funds missions to explore the World Engine's wreckage, and 18 months after it's destruction, the Kryptonite nugget (nicknamed "Emerald City" by Lex) is finally found. Lex then proceeded to retrofit the smaller Kryptonite sliver in his possession into an innovative scalpel's blade, which he used to slice off the fingerprints from General Zod's corpse.
Despite Senator Finch placing an embargo on the large Kryptonite nugget from the Indian Ocean, Lex managed to ship the xenomineral to Metropolis, and enlisted the armed guard of Anatoli Knyazev and his men to transport it to LexCorp Industries. Batman oversaw the shipment from afar, and shot a homing beacon at the truck to track it. Later, Batman arrived in the Batmobile to steal the Kryptonite nugget, dispatching the guards and chasing after the truck. However, before he could steal the Kryptonite, Superman himself arrived and swiftly stopped the Batmobile in its tracks. Afterwards, an undeterred Batman infiltrated the LexCorp building and stole the Kryptonite, leaving behind a batarang as a symbol.
At the Batcave, Bruce Wayne then uses high-powered lasers to cut through the xenomaterial. He thus transmutes the Kryptonite into a sharp Kryptonite-tipped spear, as well as three grenades of Kryptonite-rich gas. During the battle between Batman and Superman, he shoots two gas grenade at the Man of Steel, which temporarily weakens him to human levels standards, allowing the heavily armored Batman to swiftly gain the upper hand. Finally, as Batman is about to impale Superman with his Kryptonite spear, Superman and Lois both manage to convince Batman to change his mind, and they proceed to become allies, agreeing to team up against Lex Luthor, the true enemy.
During the battle against Doomsday, Superman retrieves the Kryptonite spear left behind at the cite of the previous battle, struggling to fly against the xenomineral's weakening effects, but ultimately managing, via a tremendous amount of determination and willpower. While Wonder Woman struggles to hold Doomsday at bay with her Lasso of Hestia, Batman fires a Kryptonite gas grenade at the monster, managing to momentarily weaken it, thereby giving the opening he needed to charge at Doomsday and deliver the killing blow with the spear, plunging it through the monster's heart. However, the dying monster, in it final moments, manages to temporarily loosen the lasso's hold on it, and impale Superman with its tremendously sharp and hard bone protrusion, killing him as well.
Characteristics and Effect on KryptoniansThe radioactive green Kryptonite xenomineral is known to be the greatest weakness of all Kryptonians, since the xenomineral's radiation is extremely toxic, potentially deadly to any of them. It does so by degrading Kryptonian cells, causing them to degenerate and decay on contact. This allowed physical damage to a Kryptonian possible, as well as extremely weakening their superpowers and invulnerability, to almost human levels. Physical symptoms of exposure are somewhat fever-like, including nausea, weakness, dizziness, vertigo, fatigue, immense amounts of pain, and loss (or near loss) of consciousness. Wounds inflicted on Kryptonians by Kryptonite do not disappear when the xenomineral leaves their presence, and only direct exposure to sunlight can instantly allow a Kryptonian to recover completely from its damaging effects and these otherwise incurable wounds. Kryptonite-rich gas can significantly reduce a Kryptonian's strength, durability, resilience and mobility, causing them to move as slow as a normal human, to lose their flight and enhanced sensory abilities, as well as to be weakened enough to be incapacitated by physical attacks by someone as strong as Batman in his Armored Batsuit (only 28 times stronger than a human). Despite Kryptonite gas' effects only lasting a few minutes at most, the weakening effects can still linger briefly, with it therefore taking a few more minutes for a Kryptonian to fully regain his/her superpowers.
- In the DC comics, Superman entrusted Batman with Kryptonite, in order to ensure that the latter would use it should the Man of Steel ever go rogue.
- In most DC comics iterations, Kryptonite can cause cancer in humans after very prolonged exposure to the element, but also can be used as a near infinite power source.