Having seen what the protagonist can do first hand, Master Ava thinks they have what it takes to become a member of her Dandelions, but she has to be sure. Unfortunately, the darkness of the Keyblade War is close at hand, and so she doesn't have time to test the protagonist with her usual methods. She has to speed things up, which in turn means putting the protagonist through a trial that they might not be ready for.
Kingdom Hearts Union Cross quests 426 through 555 show this trial in action and introduces the protagonist to Skuld, one of Ephemer's closest friends. She gets caught up in the shenanigans when tensions between the Foretellers finally explode. This writing will act as a recap and explanation of these Kingdom Hearts Union Cross events, so expect spoilers ahead.
Beast Castle
After recovering from their fight with Master Ava, the protagonist heads to Beast's Castle for the first time on Chirithy's suggestion. Here, they find that Beast struggles with the same negative emotion Ava warned the protagonist was in their own heart. The protagonist helps Belle and Beast clear the Heartless that Beast's bitter loneliness and Gaston's envy attracts to Beast castle. When Gaston's emotions escalate into attempted murder, the protagonist destroys his resulting Enraged Elk Heartless, and Gaston has a fatal accident. Belle's love then restores the Beast's humanity along with that of his servants.
Skuld
The protagonist makes their way back to Daybreak Town for a break after dealing with a series of tiring missions, like traveling into a dark corridor to kill a Darkside on their own. Chirithy, having assigned the missions on behalf of the protagonist's Foreteller, tries to encourage them by claiming their efforts are keeping their Union ahead of the others. It's at this point, Skuld makes her appearance, asking Chirithy why getting ahead matters if all the Unions share the same goal.
Skuld introduces herself, but to the protagonist's surprise, she already knows who they are through Ephemer. He used to be a member of her party some time ago but quit. Last night, however, he visited her in a dream and told her to find and follow the protagonist. She doesn't know why, though. When the protagonist explains that Ephemer also appeared in one of their dreams and told them to go to the clock tower, Skuld suggests that they do just that, even against Chirithy's protests. Before Skuld and Chirithy can finish arguing over it, though, the sound of an explosion catches the group's attention, and they head to check it out.
Foretellers' Clash
When the group reaches the source of the sound, they see Master Invi and Master Aced trying to kill each other and destroying Daybreak Town in the process. Skuld explains that this kind of situation is why she wants to go to the tower. In her dream, Ephemer told her that the world was going to end, and she can't think of a better catalyst for it than the Foretellers attacking each other. If Ephemer is sending them to the tower, then he must believe there's an answer to saving the world there. The protagonist ponders this and then voices for Skuld to follow them.
A Nightmare
While en route to the Waterway, Skuld and the protagonist are impeded by a new Heartless called "Darklings." Three of them emerge from a Lanes Between portal, and in broken speech, demand that Skuld and the protagonist handover their Lux. The duo clobbers them in response, but when they try to chase down the fleeing Heartless, Nightmare Chirithy materializes to block them.
Chirithy immediately notices Nightmare's change in appearance. Where it's fur was once just a darker grey, it's now a rich purple, signifying that it's wholly embraced the darkness. Nightmare Chirithy revels in this fact and then suggests the rest of them do the same, exactly like the Keyblade wielders they just fought. They can no longer use the power of the Book of Prophecies' Medals, but by succumbing to their own darkness, they can now fight independently.
Chirithy is horrified by its counterpart's complete disregard for the Master of Masters teachings and demands to know who Nightmare's wielder is. Of course, Nightmare Chirithy doesn't give a straight answer and only claims that its wielder is closer than Chirithy thinks. Then it takes its leave.
The Darklings' ability to "speak" may confuse some as to whether they're actually Heartless. The short answer is: yes, they are. Players can tell by their eyes. If a Heartless' eyes are visible, they're always yellow. Darklings are a special kind of Pureblood Heartless that occurs when a Keyblade wielder becomes one. Because of their strong hearts, they maintain some semblance of self, just like Sora and Apprentice Xehanort when they turned into Heartless. As for why Xehanort and Sora didn't become Darklings: they're both anomalies. The two of them discarded their bodies, whereas Nightmare Chirithy implies that Darklings just allowed the darkness in their hearts to consume their entire being
Your Friend is Dead
With no way to follow Nightmare Chirithy, the group continues to the tower, having their nervous Chirithy guide them through it. Inevitably, they find themselves in the tower's final room, the Foreteller's Chambers, but no one's there. Anxious, Chirithy suggests the group leave. They attempt to but get cut off by Master Ira, who chastises Chirithy and questions if the group's appearance is related Ephemer's, whom he caught trespassing the other day. At first, Ira's mention of Ephemer makes the group think he can help find him, but Ira crushes any hope of that.
He tells the group that Ephemer gathered Lux for reasons that didn't align with his and that Ephemer only befriended them to gather information he could use to undermine their respective Unions as well. He was a spy, and for that, Ira destroyed him. After a stunned silence, Skuld is the first to blow up. Appalled, she demands to know how Ira could do such a thing. Chirithy realizes that she's escalating the situation and intervenes to try and convince Ira to blame everything on it. However, the protagonist stops the Dream Eater and steps forward to take the blame themselves.
The protagonist voices that trespassing was their idea, and they did it because, like Ephemer, they're tired of doing everything the Foretellers tell them to without question. They admit they didn't know Ephemer for very long and that by coming here alone, he actually lied to them on their first meeting. However, they don't believe Ephemer was anything that Ira claims. They think Ephemer really was their friend, and they're angry that Ira took him away. For that, they have no other choice but to fight Ira because they feel like Ephemer would do the same. Ira accepts the challenge, and the protagonist loses.
Recruited
Instead of destroying the protagonist, Master Ira congratulates them and then transforms into Master Ava before healing the protagonist's wounds. Ava apologizes for tricking the protagonist, as the Ira they fought and even the room that they fought him in were illusions she created. Along with that, Ephemer is fine, but what he told Skuld and the protagonist about the world coming to an end was true.
Worse yet, said apocalypse is happening soon, so Ava needed to test both the protagonist's strength and resolve to see if they were worthy of becoming a Dandelion, a member of a group of Keyblade wielders that the Master of Masters instructed her to collect. The Dandelions won't participate in the Keyblade War and will therefore ensure that a generation of Keyblade wielders survives the end of the world to continue combating the darkness. Ephemer will lead the Dandelions in her stead once the war is over, and she would like the protagonist and Skuld to join, as they've proven to have a strong aversion to darkness.
Skuld happily accepts, but the protagonist hesitates, as they want to know what will happen to those not chosen to be a Dandelion. Ava admits that everyone not picked will inevitably participate in the Keyblade War and disappear with the rest of the world. Unexpectedly, the protagonist asks her for time to think about their decision. She honors it but only asks that they keep what they learned here to themselves.
Reasons Why
Back in Fountain Park, Skuld listens as the protagonist explains their hesitation to Chirithy. While they would like to see Ephemer again, they're not okay with leaving everyone else to suffer alone. Skuld admires the protagonist's reason and thanks them for giving her a new view on things. Still, she also has to thank Ephemer for introducing her to them and for believing she would be fine on her own when he, and eventually the rest of her teammates, left her party, and she can only do that as a Dandelion. The two shake hands, and for the moment, go their separate ways.
Fly Far, Far Away…
Later, Master Ava stands in Fountain Park when she gives her Dandelions, including Skuld, one final address. She tells them that they'll be training in a place made of dreams (the realm of sleep) to prepare for the upcoming Keyblade War. The training will help them to escape from the world when the war breaks out. She doesn't know how far she'll be able to take them before she disappears, but she pleads for them to remember not to fight, as they'll be the last wielders left when everything is said and done. With that, the questline comes to a close.
Kingdom Hearts Union Cross is on IOS and Android.