Genesis: Kingdom Hearts

Kingdom Hearts: Downfall & Redemption takes place in a possible universe after the events of Kingdom Hearts 3. Xehanort has won the Keyblade War and years later a young 15-year old boy must stop him. Along the way, he meets characters from classic Disney films, Final Fantasy games.

Pre-Downfall and Redemption
Sometime before the Keyblade war Riku and Kairi did the Right of Succession with two children. Sora decided not to, because he was sure they were gonna win. However, despite all of their efforts, the Xehanort and his darkness were too strong and the guardians of light failed. Sora's heart was fatally damaged and sought refuge in a very young boy named Nero Hikari. Nero allowed Sora's to stay and mend inside of him. Riku and Kairi go into hiding.

Story
Many years later, Nero, Killua and Kyla decided they were gonna set off from their world and explore. Unfortunately the night they were supposed to leave, darkness envelops their world and the heartless show up. However, rather then fearing it Killua accepts it, and he and Nero are engulfed in Darkness. But a radiant light envelops Nero and a "strange weapon" appears in his hand. The word Keyblade rings in his head. He searches to find Kyla but instead he finds her good luck charm on the ground. He then fights a gigantic heartless and defeats it. However he could not save his world from being engulfed in the darkness.

Nero awakes in Radiant Garden and is found by Leon. Leon explains to Nero everything about the Keyblade War, Sora, and Xehanort. Nero then decides that his friends probably ended up in other worlds like he did and that he would go find them, however the conversation is cut short by a heartless attack. Nero then meets two more people, a rabbit and a dog. They defeat a wave of heartless and introduce themselves as Oswald The Rabbit and Max Goof. They tell Nero that they are looking for people named Riku and Kairi, Nero tells them that he is looking for his friends. They decide to travel together and go off to travel the worlds in search of their friends.

After traveling through many worlds, Nero meets Killua on a trip back to Radiant Garden. Nero is relieved to see Killua is okay, however Killua seems very distant. To Nero's surprise Killua is able to snatch the Keyblade away from Nero before giving it back willingly. Nero and Oswald argue if Killua can come with them but Killua leaves. Nero is content knowing that Killua is okay, not knowing he is being manipulated by Marluxia.

After traveling to more worlds, they reach a world based on the Treasure Planet movies and meet Jim Hawkins. During the trip to find treasure planet, they meet up with Killua again and find out they are on the same mission. But rather then joining them, Killua attacks Nero and isolates him from the group. Killua reveals he is working with Marluxia and has the power of darkness now. Nero and Killua fight, with Nero being the victor. Killua flees and Nero reunites with the group, upset that his bestfriend has turned on him.

The trio then travel to Castle Oblivion, where they once again meet Killua. Killua explains that he is true Keyblade wielder and proves it by stealing the Keyblade from Nero, claiming that he is "unfit to wield such a weapon anyway". Max and Oswald apologize and go with Killua as their mission was to "find Riku and Kairi with the boy who wields the Keyblade". Nero was distraught at first but realizes he can't just back down, and goes off to find Killua again.

Nero finds Killua, Max and Oswald. When Nero refuses to back down Killua strikes him, but is deflected by Max and Oswald who claim that they couldn't just betray Nero like that. Due to Nero's strong will, determination, and heart the Keyblade chooses Nero over Killua, making him the true Wielder. The trio defeats Killua, forcing him to retreat. They then find Killua again who is being controlled by Marluxia. Nero fights him 1 on 1 and manages to defeat him. They find Kyla who was trapped in Caslte Oblivion all along and retreat to Radiant Garden.

Nero tells Kyla that he has to go save Killua. He gives Kyla her good luck charm back but she tells him to keep it, as it will bring him great luck. He promises to come back for her one day and the trio sets off to save Killua.

The trio go back to Castle Oblivion and find Marluxia instead of Killua. The trio fights Marluxia and is defeated, but before he can finish them off Killua's spirit appears and stands in Marluxia's way. Killua apolgizes for everything before disappearing, telling Nero to take care of Kyla. They then fight Marluxia again and defeat him. He then egulfs the room in darkness and appears in front of a large angelic Nobody stands behind Marluxia wielding a gigantic version of his Graceful Dahlia. Max and Oswald being unconscious and Nero on his last legs, Nero somehow calls on an inner power in himself and summons a second Keyblade. Not understanding how he did it, he charges at Marluxia and defeats him. The trio then set off to find Riku, Kairi, and Killua.