For whatever reason, the Master of Masters made Luxu his right-hand man. From all of his apprentices, the Master gave Luxu the most extensive role in his grand plan, bestowing upon him and telling him to use the Gazing Eye Keyblade to record multiple lifetimes of events into the Book of Prophecies. On top of this, Luxu was the only apprentice made aware of the Master's plot. He was seemingly filled in on everything while the others were kept in the dark, and this plan of the Master's is about to upend the Dandelions' lives.
The major events of Kingdom Hearts Union Cross quests 872 through 910 take place five years before the protagonist and Ephemer investigate the new world currently infecting Data Daybreak Town. These quests introduce a new plotline to Kingdom Hearts Union Cross that involves Luxu learning about the Master's intentions. Up until this point, the Master of Masters' cunning has only been relayed by proxy. Luxu told Ava about it during the "Traitor and the Lost Page" portion of the Keyblade War. However, in this sequence, the Master personally shows his cutthroat personality. This writing will be a recap and interpretation of these events, so spoilers ahead.
The Master's Keyblade War
The Master of Masters has a story to tell Luxu, if he would only listen. Shortly after bestowing him with the Gazing Eye Keyblade, the Master summoned Luxu to the clock tower's control room. He wants to tell him how the Keyblade War started back when he was a kid, but Luxu's astonishment at the topic currently has him answering a litany of questions: Yes, he used to be a kid like everyone else. Yes, there was a Keyblade War in his time. No, he didn't have a master, and the fight was between light and darkness. Yes, there were other wielders beside him — sort of; they didn't really count.
The Master eventually puts his foot down when Luxu asks what form Darkness took back in his day. However, he does still indulge Luxu by resuming his tale with the answer to Luxu's question. The darkness took a human shape but was monstrous, and it was when it took this form that the Keyblade War started.
Here, the Master clarifies that the Keyblade War he told Luxu about — the one they're currently preparing for — isn't about to begin, it's just continuing. The battle with Darkness that started during his childhood never stopped, because Darkness has the ability to change its shape and form, and can even hide in humans. In other words, it can't be destroyed. Luckily neither can the light, and if things go according to the Master's plan, the war may finally reach a definitive conclusion.
Said plan revolves around the Foretellers' Keyblade War, which the Master plans to use as a way to destroy the current world. Once that's done, Master Ava's Dandelions will journey out to the world's splintered remains and gather the light necessary to rebuild the planet. This plan will take place across three stages; "X [Chi]," "unchained," and "Union Cross."
The Three Stages
Of course, the terms "X," "Unchained," and "Union Cross" refer to the three iterations of the Kingdom Hearts Union Cross game. However, in-game, the Master uses them to represent the three stages his chosen Keyblade wielders will go through to fulfill his plan. At least, this is what Master Ava implies with the way she describes the "Unchained state" in the "Search for Ephemer" questline.
Master Ava uses the word to describe Ephemer after recruiting him to the Dandelions. For wielders to become Dandelions, they must prove to Ava that they're independent of Union loyalties. These wielders must prove that they're willing to put their fellow wielder first, regardless of whether said wielder belongs to the same Union as they do. In doing so, those wielders make themselves unbound, or "unchained," from any one Union — unlike most Keyblade wielders. With this in mind, here's what each of the Master's three steps means:
"X" refers to the stage where the Foretellers unknowingly recruit Keyblade-worthy hearts for Ava to pick out for her Dandelions. These wielders tend to put their Union above everyone else, and end up as cannon fodder for the Keyblade War because of it. "Unchained" refers to the wielders that do manage to become independent from their Union. Ava selects and tempers these wielders into Dandelions. Finally, "Union X [Cross]" refers to the missions that provide the light necessary for the Dandelions to rebuild the world after the Keyblade War destroys it. Once the world is rebuilt, the Master somehow plans to call a truce with Darkness.
The Cornerstones of The New World
Luxu isn't all too comfortable with the Master's plan. For one, the Master claims he wants to kick back and take a nice long rest while the Dandelions do all of the work, and Luxu doesn't like the idea of leaving fate in other people's hands. Also, the Master claims that once the Dandelions complete their mission, they'll become the cornerstones of the new world. In response to Luxu, wondering if this means that the Dandelions will be able to return home, the Master of Masters gives him an unsettling reply: "Maybe not all of them."
What the Master means is clarified in the very next cutscene. In it, the Union leaders will discover that they've been completely cut off from the real Daybreak Town. In other words, they're stuck in the Data World with no discernable way to get back to the real one, not even after they restore it.
Luxu's need to ask if the Dandelions will be able to "return home" implies that the Master told him they would end up cut off from the real world in a previous conversation about his plan. The Master's ominous response is proven by future Kingdom Hearts games where characters like Ventus and Lauriam are active in the real world, while others like Ephemer and Skuld are nowhere to be found — yet.
Kingdom Hearts Union Cross is available now on Android and iOS.