Upon learning the Dandelions fight with Darkness was hopeless, Ven rushed the entity head-on by himself. The replacement leader reasoned that if Darkness could use the pure light of his heart to give itself an indestructible form, he could use it to give it the opposite — a physical vessel that the Dandelions could actually destroy. Whether Ven's actions worked remains unclear, as the clash resulted in a burst of blinding light.
The "Backup Data" cutscene of Kingdom Hearts Union Cross quest 978 reveals that Ventus wasn't the individual Luxu sent into the future. It is the first of any quest to contain no gameplay and the second of the last three quests in the Kingdom Hearts Union Cross storyline. It is also the first quest of the game's two-part finale. This writing interprets and recaps its events to clarify the Union Cross story.
Backup Data
Ephemer steels his belief that Lauriam doesn't blame Ven for Strelitzia's death. Ven fell unconscious following his fight with Darkness, and Lauriam's currently alone with him in his room, checking on him while Ephemer, Skuld, and the Union Cross protagonist wait outside, and even though Ephemer allowed the scenario, it shook his resolve for a moment. Skuld and the protagonist have no problem entering Ven's room to make sure everything's alright, but Ephemer's decided to trust Lauriam.
After a moment of silence, Skuld questions if the others think the world is really ending, to which Ephemer admits he's unsure. They don't know enough about darkness, the Keyblade Masters, or about the data world itself. That said, he reminds Skuld of Ava telling them about how worlds taken by darkness simply fall asleep — along with their inhabitants. The idea gives him hope because he helped organize the Dandelions, so he'd feel like a failure if they all ended up destroyed.
Skuld reassures him, though. The Dandelion's safety isn't just his responsibility. Still, if he decides what they all do next, she speaks for everyone when she says that he has their full support. The sentiment inspires Ephemer to thank Skuld for always having his back, and as the two of them and the protagonist enjoy the moment of sincerity, Elrena approaches the group from behind.
Meanwhile, Lauriam apologizes to Ven's unconscious form. He found it easier to blame him over Strelitzia's death because he could actually hurt him, unlike the abstract concept of darkness. He admits attacking Ven with such reasoning was wrong, and the only explanation he can give him for doing so was that he couldn't manage his anger over the way Strelitzia died.
That said, an apparition of Strelitzia finds itself holding Lauriam back. Ven claimed his actions would make Darkness a part of him, and Lauriam can't help but wonder if that's true. If it is, it would mean Ven has become Darkness' physical form, and Lauriam could hurt it through him, so Strelitzia appears and grabs his wrist from behind, telling him to let go of his negative emotions because they play into Darkness' hand. Lauriam whirls back to see her, but it's like she was never there to begin with.
But he's not completely out of luck. Ephemer peeks into the room and informs him of Elrena's arrival, and when he joins the group outside, Elrena reveals that her Chirithy thinks it saw Strelitzia a few days ago. It believes it saw her walking alongside someone in a black coat (Luxu). Chirithy can't be sure, though, because Strelitzia was wearing the same white coat she wore in Lauriam's dream. It only thinks it's her because it saw her distinctive hair resting outside her hood.
Unable to make sense of the situation, the group goes to see Brain about it in the Foreteller's Chambers, and he manages to come up with a theory. They all know a black coat can only mean the Master or Luxu, but the Master's been missing for longer than Luxu, so it's likely not him. As for Strelitzia, he believes Elrena's Chirithy saw a data copy of her. The data world exists as a copy of the original and its inhabitants, as evident by Ephemer's Shift Pride.
Here, the story seems to experience a plot hole. Lauriam's surprised at the possibility that digital copies of them all exist, which doesn't make sense because he's the one that told Ephemer about them in the first place, thus leading to the latter using them to fulfill the Master's Shift Pride regulation. To give Kingdom Hearts Director Tetsuya Nomura the benefit of the doubt, here's a possible explanation for Lauriam's reaction:
Lauriam used to believe that data copies of the Dandelions existed, hence why he checked the data world for them just before the Dandelions were permanently moved there. When he didn't find any copies, he figured he was wrong, and since he wasn't there for the Shift Pride meeting, he didn't know that Ephemer found a way to activate the copies and was subsequently using them to fulfill Shift Pride. Simply put, Lauriam's questioning Brain's theory because he acted on it already, and it turned up nothing.
But Brain knows digital copies exist, and since they exist for regular Dandelions, they exist for Union leaders, including Strelitzia. His working theory is that someone found and accessed Strelitzia's data back when the data world glitches started happening, and that's why Elrena saw a glitch of Strelitzia in the warehouse. Who accessed Strelitzia's data and why — Brain isn't sure, nor can he answer Lauriam's question of whether they can use Strelitzia's data to bring her back.
Ultimately, the group decides to head down to the basement lab. If they believe Darkness, then using the lifeboats in the lab to return to the real world will destroy the data world. However, the group decides to take the risk because it believes they'll find more answers about Strelitzia and her mysterious companion in the real world. As they're leaving the room, though, Brain pauses to look back at the Book of Prophecies on his desk.
Kingdom Hearts Union Cross is on IOS and Android.