So Master of Masters (MoM) is supposedly from an entirely different Worldline (think alternate dimension/Universe) and has this conflict with an entity called Darkness who essentially destroyed his world and everyone he knew. So MoM somehow travels to a new Worldline as he builds up this grand plan to destroy Darkness for good. He eventually takes on 6 disciples the Foretellers/Union Leaders.
He gives each of them a Tome of Prophecy except Luxu who he entrusts with another task. In 4 of the Tomes he removes a page that tells of a traitor amongst them. The Foreteller with the missing page is tasked with finding this traitor. Shortly after MoM disappears leaving the Foretellers to essentially turn on each other which causes the Keyblade War which destroys the world.
One of the Foretellers trains a bunch of Keyblade Wielders herself forming a group called the Dandelions which is basically the last hope there is. Eventually the Keyblade War reaches its climax and nearly all the Keyblade Wielders are killed.
Come to find out, this world is a data world so the surviving Wielders have to escape the Data World to survive. Marluxia and Larxenes original selves are some of the survivors and are successfully sent to the real world. Another survivor called Brain is met by Luxu who transfers his heart into Brain.
While this is going on your player character and two other Wielders are ambushed by Darkness, which isn't a singular entity but multiple. Your character is seemingly possessed by Darkness and attacks the other two Wielders. Turns out this was all a trap and several of the Darknesses are trapped in a pathway with the Player Character while the other two escape. Player Character dies and reincarnated as Xehanort.
Meanwhile Luxu watches the destruction of the world.
An undetermined time later Brain wakes up in Scala Ad Caelum and is met by some robed hooded figure with a hat on named Sigurd, they talk and yadda yadda.