This story happens when something is destroyed, and this story happens when something is protected. It's the paradox that allows it to happen. Please remember that, when you wonder how the knight can have lived the life she did. Please keep that in mind, as you've watched her destroy herself to save everyone and convince herself that she failed to save anyone in the end. Please recall: this is a story that happens when something is destroyed in a way that saves it.
Rin is safe now, and so Saber unleashes the power of Excalibur on the twisted mass that is the Holy Grail she's sought for so long. It's too much power. She has none left. The cascade of light consumes her and pulls her from this world.
She destroyed it to save all the ideals she fought for, and she lost herself in the process. That's how it went. In the moment of that paradox, two different concepts align in the world beyond this world. They are the Grail itself, and the power behind the sword that destroyed it. Not the sword itself—that came, that destroyed, and that went. But the sheath that held it, the ideals that supported it…Avalon crosses paths with the Grail.
Arturia Pendragon is a being who has defined herself by the ideals within Avalon and the pursuit of the Grail, so she may exist in this space, this shimmering white glow scented faintly of apples. Around her, light crashes so hard that it makes a sound like water falling in a roaring wave. And another whiteness slowly forms into its own shape.
Saber isn't the only being who's ever existed by the grace of both the Holy Grail and Avalon. There is one other.
Her hair forms first, strands of perfect white separating from the waterfall around her. But her smile comes next, purer than any light—
At least, that's what Saber thinks, if she can remember how to think. She knows she'll disappear any moment now. She's made what peace she can with her existence and her end. The wish she yearned for was wrong; if the fight between Shirou and the man that Shirou became taught her anything, it taught her that. There is no end for someone like her but to accept the pledge she made so many years ago and move forward again into her final moments. So why would the world taunt her like this? Why would it show her one of the greatest regrets she has, at the moment when she's vowed to push past her regrets?
The woman's body has almost formed. Only her eyes are still hidden by the light. But still she seems to know what's around her. "Saber…it's you, isn't it?" Her voice is easy and delicate, with none of the pain Saber knows she should feel.
"In this place, it could only be me," Saber says. And then, because she can say nothing else, she adds, "My princess."
Irisviel von Einzbern opens her eyes and smiles freely. "I see! That's how I knew, my knight."
Saber wants to run forward to cover the few steps between them. She doesn't dare. "Why…? I was done. I failed you, as I failed everyone…but I chose not to dwell on it any longer." She realizes, to her despair, that she's angry. She's angry at her princess for coming here. "Why?"
It must have shown in her voice, because Irisviel bows her head with an apologetic smile. "I'm sorry, Saber. I brought you more pain, and that's terrible, isn't it?"
"No," Saber says quickly. As much as she wants to see Irisviel smile, she doesn't want to hear her apologies. She's the one who should be apologizing. "It isn't your fault you're here. The Grail—"
Irisviel is shaking her head. "My, you're still a little silly, Saber. Don't you understand? I chose to come here because it's the only time I could. So let me apologize if it hurt you."
Saber lost the ability to cry a long time ago, so she merely stares, uncomprehending.
"Oh, good!" And Irisviel giggles lightly. Saber realizes now that somehow, she still has a heart, because it's skipping and flailing in her chest at the noise. "Good, you're speechless. So I can say it now. I can say something I forgot to say to you ten years ago…it's been bothering me since then."
Saber knows that the woman in front of her is simplifying. Neither of them has quite known the passage of time in the past ten years. But still she listens raptly. After a moment, she realizes that her voice has returned, just a little. "I will listen to anything you say, Irisviel."
Irisviel smiles. It's such a sad expression, but for the first time it strikes Saber that it's sad for neither Irisviel herself nor Kiritsugu, for whom she smiled sadly so many times ten years ago. It must be sad for Saber. "I know you will, Saber. So please, my knight…believe me—" And in a few light steps, she crosses the brilliant space between them and folds her soft hands around Saber's, around Saber's hands that are only used to fighting. "Believe me when I say this comes from my heart. Isn't it funny? There was a time I didn't know I had a heart! Kiritsugu found it for me, and I'm so grateful he did, because now I can tell you something."
"What…?" Saber has almost lost her ability to speak again.
Irisviel still smiles at her, and now Saber feels like the warmth in that smile is being transmitted to her through the hands that hold hers. "Thank you, Saber. Thank you for being my sword at my side. Thank you for protecting me."
No. No. No, Saber can't stand for that. Even if she can't cry, she can burst out as if she could. "But I failed! I cannot take your gratitude, my princess. It's meaningless. I failed to protect you!"
The warmth in Irisviel's gaze suddenly transmutes into burning anger. "How dare you say that, Saber! How dare you tell me my gratitude is meaningless!"
Saber stops. There's nothing she can say now, because it's true: she has no right to tell Irisviel that any emotion of hers is worthless. Everything about her princes is precious.
The smile does not yet return. Irisviel's gaze is still fierce. "Does it matter if you failed in the end? It still happened, you know. You still protected me as much as you could." Now that gentle smile does spread across her face again. "I'm still happy…because I got to see such a beautiful knight fight for me."
There is only one thing Saber can say. It's a name. "Irisviel…!"
"I'm special!" Irisviel laughs again, giddily. "I thought I was alive because I was loved by Kiritsugu. But I was more than alive, because I was protected by the king of knights. No matter what happened, I'm so grateful I can say that."
Saber feels something wet on her cheeks. Is it possible? "Then…"
"Will you say it, Saber?" Irisviel lets go of her hands. The sudden solitude of their bodies is more of a shock to Saber than it should be. But it doesn't last. Irisviel takes hold of Saber's face. "Will you say that you protected me, and you are my knight?"
"Of course." Was that all she needed to do? It seems so silly. But it's what Irisviel wants. "I am your knight, Irisviel…and I always protected you."
"Oh…" They're fading. The alignment of the Grail and Avalon is almost over, like an eclipse that passes without ever quite being full. But Irisviel leans forward. Her lips brush against Saber's, and Saber feels them open to speak. "Thank you, Saber."
And as they stand there, the world above and below tries to part them, but this is not the kind of story where a princess and her knight can be fully parted. So instead it breaks them down together, and they both vanish into the light, Irisviel's hands still on Saber's face.