Luxu now possesses the Master's box and knows the purpose behind the lifeboats. The Master alludes that the box's contents will help the Dandelions retain their bodies when they get to the future, while the boats will destroy the world after the last departs for the future. With these nuggets of knowledge, Luxu prepared to go about fulfilling his role, believing he heard everything the Master had to say. He was wrong.
Quest 979 is the final quest in the Kingdom Hearts Union Cross storyline, and its cutscene "7 and 13" sees the Master of Masters finally reveal to Luxu the twisted truth behind his plan to defeat Darkness. This writing is the first entry in a four-part series covering quest 979, and it acts as an interpretation of the events of cutscene "7 and 13" to clarify the Kingdom Hearts Union Cross finale.
7 and 13
Before Luxu departs, the Master has one last thing he needs to tell him, something that he will not like. So — to ensure that his apprentice at least tries to hear him out, the Master reiterates how Darkness cast aside its physical form when he was a child. This is important because while the act made Darkness indestructible, it also limited how long it would retain its intelligence. So now it spends its time attempting to possess people's hearts to steal their bodies, something that has admittedly worked on a few of the Master's friends.
When Darkness discarded its original body, it became 13 beings. In their quest to find tangible forms, these Darknesses inadvertently created the Heartless, but while the Heartless are tangible, they are mindless, so the Darknesses continue pursuing people with strong hearts. Herein lies light's opportunity to defeat it. Darkness loses its invulnerability while in human vessels, meaning light can bait it into vessels with strong hearts to destroy it. For this reason, the Master raised Luxu and the Foretellers as his apprentices — They're his bait.
Luxu has the expected reaction: shock — but the Master doesn't want him to misunderstand. He didn't raise them all as simple vessels. He raised them to have hearts so powerful that Darkness can't steal their free will, even if it enters their bodies. This way, they and the Master can each absorb one of the seven strongest Darknesses among the 13, go their separate ways to split them up, and then destroy them within the Station of Waking. He's trusting his apprentices with this because he believes they're the only ones capable of pulling it off, and he can't waste the opportunity for light's victory on emotions.
The Master will instigate the Keyblade War between the Foretellers to achieve this plan. When he tells them of a traitor among them, they'll start to feel doubt for one another, as well as other negative emotions that Darkness feeds off of. The physical confrontations these feelings will generate will amplify the negativity, convincing Darkness that all seven of them are worthy vessels. That leaves six entities. Five of them will be sealed away within the new Union leaders, while the final Darkness will be sealed away within the data world when the lifeboats destroy it.
Hearing the inexperienced Dandelions will be involved is what sets Luxu off; he has no idea how the Master can expect him to be okay with sacrificing so many people, so the Master tells him again: They can't let emotions stop them from ending Darkness once and for all. The Master has seen what happens to those who lose their hearts to it. It's a terrible way to go, and they have a chance to spare humanity from that fate. That's all this is about. It's not about sacrifices or acting out some heroic tale. Besides, he's not sacrificing anybody. He has a plan to keep them all safe.
Luxu struggles to come to terms with this because the Master's plan does indeed kill them all. If they destroy Darkness while it's bonded to their hearts, their hearts will shatter in the process, and they won't survive it. Birth by Sleep confirms this when Ventus destroys Vanitas, the original Darkness he ended up absorbing. The process nearly erases Ven from existence, and he only survives because Sora allows him to use his heart to remain whole until he fully healed. Everyone involved in the Master's plan doesn't have a "Sora" to rely on like that. There's also all of the weaker wielders who will simply die during the Foretellers' Keyblade War.
Regardless, Luxu swallows his feelings about the situation, motions that he understands, and then leaves the room without another word — which, of course, is another reaction the Master expected. He knows Luxu's not going to fully commit to his plan. "May your heart be your guiding key" is the Master's fancy way of telling his apprentices to follow their heart, which in and of itself is just another way of saying do what you believe is right. The Master suspects — counts on, really — Luxu will do just that. He is, after all, the traitor mentioned in the Book of Prophecies and more than likely the way the Master plans on pulling this off without permenately killing everyone.
The Traitor Luxu
The Master's revealing of Luxu as the traitor puts a few things into perspective. Luxu likely realized the Lost Page referred to him as a traitor long before he confronted Ava during the Keyblade War cutscene "As The Bell Tolls." He also likely realized that the Master wanted him to betray him, as he claims to believe that he finally understands the Master's intentions during quest 977's "A Seed of Light" cutscene.
Union Cross heavily implied Ava to be the traitor as she came the closest to actively defying the Master, giving Brain the Book of Prophecies instead of Ephemer. However, Brain never read the Book and gave it to Ephemer anyway, so Ava ultimately didn't succeed. She was also a suspect because her strike against Luxu tolled the bell that heralded in the Keyblade War. Gula believed that the Lost Page hinted that the traitor would be the one to cause the bell to ring, and he was technically right. Luxu goaded Ava into striking him by telling her the Master's plan, which he essentially admitted to in Kingdom Hearts 3's secret ending.
Kingdom Hearts Union Cross is on IOS and Andriod.