Aced has declared open war on the rest of the Foretellers. The Master of Masters told him to step up and take the title of "leader" if he ever felt that Ira wasn't up to the task, and he has decided to go about doing so using sheer force. Not only that, once he wins, he plans to banish the rest of the Foretellers, take over their Unions, and unite the world under his rule. The protagonist, Skuld, and Chirithy are scrambling to find a way to stop this, and they think the answer lies with the currently missing Master Ava, who they believe Master Gula may know the location of.
This writing covers and explains the second part of the Keyblade War. Kingdom Hearts Union Cross hosts the event as a series of quests told across three parts. These quests are separate from the main Kingdom Hearts Union Cross storyline for meta reasons, as what happens in them is wiped from the minds of everyone save for a select group of individuals. However, everything that occurs within them is still cannon. Spoilers ahead.
Gula and The Lost Page
Gula greets the protagonist, Skuld, and Chirithy when they arrive at the storage house he likes to frequent. The fact that they're not collecting Lux makes it obvious to him that they're associated with the Dandelions, and as such, must be looking for Ava. He explains to them that Ava went out looking for Master Luxu, their fellow sixth apprentice. Luxu disappeared right after the Master of Masters, so Ava's hoping he'll be able to tell her where the Master went, as he's the only one with the power to put a stop to the war.
Gula himself doesn't have any hope that the war will stop, though, because he failed to carry out the role the Master assigned to him. He was supposed to use and interpret something called the Lost Page to find out who the traitor among the Foretellers was. The Lost Page is a sheet from the Book of Prophecies that the Master gave to Gula. The Master told Gula that the Page was ripped from the Books of every other Foreteller, so as far as Gula knows, he's the only one alive in possession of it.
Gula was supposed to use the contents of the Page to find out who the traitor among the Foretellers was because the traitor is the catalyst of the Keyblade War. Essentially, the Master gave Gula the hardest task of the Foretellers. Gula thinks that the Page pointed to Master Aced, but Gula failed to stop him from going through with his plan to take over the world. Even if he succeeded, though, it might not have stopped the war because Gula's not a hundred percent sure that Aced actually is the traitor.
Lost Page Translations
Gula admits he's not supposed to be telling the group anything about the Lost Page, but the world's going to end, either way, so he doesn't see the point in keeping secrets any longer. As such, he recites passages from the Page for the group to hear: "Imbalanced observed, strength misplaced, a future filled with sorrow," and "Words of truth misunderstood as they explore the secrets of tomorrow." Exactly what these mean is a mystery, but educated guesses can be made.
"Imbalanced observed" points to the Foretellers' discovery of Nightmare Chirithy, which in turn leads them to think there's a traitor among them. "Strength misplaced" refers to the Foretellers using their Unions to gather more Lux than the other. "A future filled with sorrow" refers to how all of these things lead to the inevitable end of the world as everyone knows it. As for "Words of truth misunderstood as they explore the secrets of tomorrow," it likely refers to the Foretellers misreading the Book of Prophecies. The Book warned them of a Keyblade War, and they used it to try and prevent said war from happening, but in doing so, they actually triggered it.
After reciting these two, Gula recites a third: "With a single strike, toll the bell and herald the end. Bring war upon us as fate did intend." As if on cue, everyone hears the bell toll outside, signifying that nothing can stop the Keyblade War anymore. As such, Gula tells the group to get back to their Unions, as their leaders will assemble them soon.
Ava and Luxu
Ava actually manages to find Master Luxu. While Gula discusses the Lost Page with the group, she finds him sitting on a cliff overlooking Daybreak Town. Having not seen him in nearly a year, she asks the obvious question of where he's been, and to her regret, he answers. The Master of Masters tasked him with watching how events would unfold once he disappeared. Unlike the Foretellers, Luxu didn't receive a copy of the Book of Prophecies. Instead, he was told to naturally make his way to the future written in the Book. Once he witnessed the end of the world, he would move onto the next.
He knows that Ava was searching for him with the hope that he could lead her to the Master, who would then put a stop to the war, but Luxu can't help her. He can't because the Master doesn't care about the fate of the world, and in fact, wants it to end. Luxu knows this because he was given a copy of the Book of Prophecies' Lost Page, and it describes the Master's intentions. Right now, the Master is doing what he needs to do to make sure Luxu can fulfill his role.
The Traitor
Ava doesn't believe that the Master would cause or allow such a tragedy to occur and demands that Luxu tell her what the Lost Page says and who the traitor is because she suspects it's him. Before answering her question, Luxu summons his Keyblade, as the Lost Page has already told him what happens next. He allowed Ava to find him just to bring about the moment. Kingdom Hearts Union Cross doesn't show what Luxu says, but judging by Ava's denial and the events that unfold after that, he tells her that she's the traitor.
Luxu suggests Ava fight in the war if she really doesn't believe him. If there's another truth, she'll find it once the dust has settled. He wonders, though, if it ever crossed Ava's mind that the Master didn't assign them their roles to try and save the world but to see how their hearts would tell them to go about fulfilling their tasks. It hadn't. In fact, the idea that the Master actually wanted the Keyblade War distresses Master Ava so much that she actually attacks Luxu, completely going against her philosophy of not using her weapon with fatal intent against another wielder.
She strikes Luxu with so much force that the shockwave generated from her Keyblade meeting his blooms out for miles. It stretches out from the outskirts of Daybreak Town to its center, where it causes the clock tower's bell to toll, consequently fulfilling the "With a single strike, toll the bell and herald the end. Bring war upon us as fate did intend," passage of the Lost Page.
Kingdom Hearts Union Cross is available on IOS and Andriod.