Refuse her
Your final whisper reaches Binah, who is tempted to huff in regard to your sentimentality. But she accepts your choice nonetheless and shakes her head.
"You shall remain on Remnant, child. Now is not the time for you to make a choice that may never be made undone."
"But, but Ash is going too, right? And there's tons of incredible stuff where you're going, you can't just say I'll never get back! You already came here once already, too!"
She makes a surprisingly good and persistent case. However, Binah is undeterred. "It is true that precedents for dimensional travel exist," she allows, "but Angela made use of a loophole to send me here."
"With assistance of Yin and Yang," Angela adds. The carps are gone by now, though the book in her hand pulses softly with their light and darkness. "They expended a great deal of the power they still had; it will not be possible again anytime soon." She steps forward to take Binah's place in confronting Ruby. "Look back at everything you would leave behind if you went with us. Do you truly accept never seeing any of it again, should the worst come to pass?"
The girl falters as her impulsive enthusiasm is undermined by rational thought. Angela's words hit deeper than she likely intended. Ruby drags her feet, cheeks blown out in a pout. The tantalising pull of secrets to be unearthed is strong after all. "What about the other way around?" she asks in a final attempt to get what she wants. "Can't you come here?"
Angela seems to see it too, going by her indulgent smile. She still has to disappoint the younger woman: "Unfortunately, relocating my Library is outside of our current means. We are still not done rebuilding and the building itself was never supposed to move at all."
The following silence lingers for a moment. Ruby deflates, so Binah decides to send her on another path entirely. "If the many secrets of our world intrigue you so," she starts with a smirk, "then you will find a number of them in yours as well. Find Weiss Schnee, Penny Polendina, Roman Torchwick, and Arthur Watts. Each of them holds pieces to the puzzle you seek to complete. The rest of what you are able to obtain, you already know where to find."
A faint motion is made toward the magical girls, prompting a curious look from Ruby. Neither of them seems quite certain how to treat this turn of events, though Love moves to take her hand. "I think Binah is right," she says. "You belong on Remnant. Let's go."
The girl lets herself be led away. Courage, Saffron, and Anemone pass through before her, their goodbyes already said. Ruby turns back at the last step, finally realising that this is indeed farewell. She lets go of Love's hand and darts back to hug Ash tightly, then awkwardly shakes Binah's hand. Angela allows it all without a word of complaint. She is surprised when the young woman shakes with her as well.
It will be the last exchange between these two dimensions. Ruby will return home downtrodden, but ultimately realise the colossal error she was about to make; remembering Binah's lessons on temperance, this moment will spur her rise to become a calmer individual. Though she pursues the leads she received, she will take her time to rise as one of Remnant's greatest huntresses by virtue of her silver eyes. Her sister Yang achieves similarly and they both will live long lives.
The magical girls returning with her remain together for a while. Love and Anemone finish their education at Beacon, each for her own purposes. Through their prowess and their magic, the four will eventually become known as humankind's guardians. Though some expect each to claim one of the four major kingdoms, the sisters will decide to form a court all their own; mediators between the realms and warriors that assist those in need. It is not the last time the King of Greed is seen, even the Knight of Despair sometimes makes an appearance; however, the Queen of Hatred and Servant of Wrath will never strike at Remnant.
Anemone remains the most elusive of the four, content to stay in the background and spread her blessing across the world. Saffron often walks the various cities, both to ascertain their peace and in search of treasure to make hers. Even with Miltia as her eventual bride, the desire for more does never cease. Melanie will often joke about it.
Love continues as the most active of the four, forever sworn to fighting Grimm across Remnant. She rarely interferes in human or faunus conflicts unless her sisters agree that the case is clear. One such matter is the White Fang; with the absence of Adam Taurus and Blake Belladonna as moderate influences, the group fully descends into extremism and terrorism. Love exterminates them over several years. Some escape as they realise their original cause is already being fulfilled, but most are slaughtered.
Courage forms the legal counsel of their little group. She proposes and sometimes forces improved regulations upon the kingdoms, removing biases against certain groups or favouring others. Many are not happy with her equal treatment of everyone, but none have the ability to stop her. In time, those who remember other times will pass and Remnant will know peace. From time to time however, those who rise in Courage's esteem will be offered to join her harem; these men, women, and others all enjoy her trust and confidence, as well as special treatment no other receives.
The first and most notable of these people is Weiss Schnee, who will go on to topple her father's Dust empire twofold. First by providing large quantities of Dust to lower prices, then by revolutionising the entire market with portable Dust generators; at the cost of one's aura, the user can create their own Dust as needed. At first it becomes a staple for hunters and effectively removes their need to resupply on missions, but soon every larger company possesses such converters. Access to teleportation and underground laboratories ensure that none will figure out how to replicate this technology for as long as Weiss lives. The Schnee Dust Company crumbles under her efforts, only to be utterly crushed by Courage's reforms. Weiss's siblings will long since be removed from the company by then, leaving only the elder Schnees to be buried when it collapses.
The rapid shift in Atlas leads to unrest and uncertainty, which Penny Polendina and her father Pietro help calm; it is but another step on Penny's journey to understanding and accepting her own nature. They are quietly backed by James Ironwood, who knows better than to make his involvement known. He will never hold an official post again, but ultimately comes around to the four guardians and backs them from behind the scenes.
The man who brought him down, Arthur Watts, fades from Remnant. He lives the remainder of his life on Menagerie under a fake name, spearheading scientific advancement and the kingdom's development. Though the faunus will first be wary of him, Chieftain Ghira's receiving him with open arms and Arthur's expertise shall eventually turn their opinions around. He will rarely waste a thought on the life he once lived or the cold grasp Salem held him in; the only one who ever hears even a fraction of his secrets is his wife, a development he himself will be the most surprised about. Most of it, including Binah and the true nature of the magical girls, he takes to the grave.
Menagerie herself will develop into a prosperous nation. Even when Ghira Belladonna retires, his daughter Blake will succeed him in leading them onward; she returned home after her traumatic experiences in Vale, never quite the same. Her fire has died, conviction broken in the face of a true monster. In its place eventually steps a quiet love of life, appreciation for the time she may not have gotten had things turned out differently. Thrice, Blake survived certain death. And so she will step up with vigour similar to her father's; the secretive human scientist working with the Belladonnas whispers advice to her whenever it is needed, displaying a great intellect that Blake will frequently make use of.
Roman Torchwick and Headmaster Ozpin will both breathe their last as living legends. Though a known criminal, Roman shall be remembered a saint. Ozpin's achievements are no less great, if farther spread out in time. His last good deed is to ascertain a partly amnesiac Amber's wellbeing; the Fall Maiden will recover from Cinder's assault and manages to find happiness in time. Once it becomes time for Ozpin to reincarnate however, he finds that the fall of Salem has ended his own duty as well. As he, too, is consumed by Remnant's light, he will speak words of thanks that Binah never hears.
But that is the far future.
Once Ruby and the magical girls are finally through the rift, it closes. A heartbeat later, the entire simulacrum shudders and wavers. The sky seems to drop and the city below shrinks. This part of the Library returns to where it belongs. Sable flutters his wings in anticipation, awaiting to be reunited with his companions.
"I saw and heard your E.G.O," Angela notes with a glance to Ash. It is impossible that she missed the similarities, though she may never have gotten a close look at the memories or replica of Angelica. "How old are you?"
Despite Angela's imposing presence, Ash simply answers as usual: "Something like fifteen or sixteen, erm, body-wise. But from what we figured out, it's more like three. Some weird stuff happened," she adds over the blunette's curious look. A glance is thrown to Binah. "Where should we start?"
"We are missing several people for this conversation," the older woman notes. She studies their surroundings as they take familiar form. The piece of her floor slots itself back into the Library. Familiar lights shine indirectly, illuminating the Floor of Philosophy. "I can not be bothered to repeat the same tale a dozen times."
"Of course," Angela agrees easily. "I have informed the Patron Librarians, they will be here shortly." The book of Yin and Yang vanishes, then she looks at Sable. "Though I am surprised you have regained your body as well, Little Bird."
"Sable," he corrects. "That is my name now."
The bird flutters on top of a bookshelf to observe, though Angela only offers him an acceding nod. "Very well. Sable, then." So saying, she throws Binah a little grin. "I am getting more curious how all these things came to pass."
"It is a tale for the ages," the other woman agrees with a cheshire grin. She settles at her table, the small pack placed on its surface. "Eight months spent in a different dimension made for a pleasant vacation, but I dutifully completed your task. Why was I made Salem's executioner, though?"
A vague motion is made toward the books, which a pair of assistant librarians is already busy carrying away. Angela glances that way and sits opposite to Binah. "You weren't her executioner," she corrects. "You were her salvation. Yin and Yang approached us about undoing a wrong they committed long ago; after drowning in the river and becoming Abnormalities, they lacked the power to do it themselves. And they only realised after Ayin completed the Seed of Light."
"How peculiar. To make me the heroine of such a tale."
"Yeah, it doesn't fit you at all!"
The voice interrupting them is familiar. Roland strolls through the doorway with his usual, cocky grin. His motions are swift and certain, at least until he notices the young woman flanking Binah. The way his eyes grow wide and he almost stumbles over his own feet are everything she hoped for. Then comes the suspicion and a trace of anger. His gaze lowers to Binah.
Another may be intimidated by such a formidable warrior marching right at them. His suit rustles ever so faintly, hands in his pockets as he grasps for the Black Silence's gloves. Ash makes to speak, but a gesture from her mentor keeps the girl quiet. They both await Roland's arrival, looming over Binah. "I'll just give it to you straight," he growls with a sharp nod toward Ash. "Is this some sort of prank? Because let me tell you, it isn't funny. At all."
The tension in his body grows notably. Binah simply smirks at him in return. "I do not enjoy a number of painful deaths, Roland. Neither is this a prank, nor a misunderstanding. Anemone, the magical girl of Justice, found her on Remnant. Without memories her own, she never knew of her origins before we all saw the proof for ourselves; a Distortion and subsequent obtaining of E.G.O do not lie, and neither does the melody that inscribed itself into her unborn soul."
Roland stands thunderstruck. His gaze flutters between Binah and his daughter, who seems similarly at a loss for words. Angela and Binah wait patiently as the two try to speak to each other, yet neither knows how to begin.
In the end, Ash wordlessly produces her flute. She sets it onto her lips and begins to play; the first notes rouse the entire floor, then she settles into a familiar melody. Roland grows stiff at first, then relaxes as a melody he can never forget becomes beautiful for the first time. Tears brim in the corners of his eyes, then slide down his pale cheeks.
The music is abruptly cut off when he embraces his daughter. Ash's flute vanishes, but she makes no attempt to push him away. After a moment, her arms slowly slide around his waist.
"I never thought...." Roland starts weakly, only to trail off. He squeezes Ash even tighter, unable to quite believe what is happening.
"I didn't, either," she answer softly. Binah keeps her huff quiet, though she spots Angela's smile quite easily.
Their embrace only looses when the other Patron Librarians arrive, alone or in pairs. Roland finally lets go, but only after taking a long look at his daughter's face. Then he rapidly turns around and sits next to Angela, facing Binah. "I want to know everything," he demands. "Start from the beginning."
"Very well," Binah agrees. She meant to do this anyway. Everyone else crowds around while Sable returns to sit on her shoulder.
"In the beginning, there was light."
These are the final words you hear; Binah's journey is complete, and so is the role you played in it. Lives were lost, potential squandered, but many were saved as well. The fate of Remnant has irrevocably shifted for the better, its foundation laid on corpses and hope alike.
And with that, your light fades.