Just a short one-shot of Regis welcoming Luna to the afterlife. And I never saw Kingsglaive, so if this veers off movie-canon I apologize.
"Luna..." the voice was distant, like an echo through a distant hallway. "Luna." a little more clarity this time. Someone was calling her name. She was aware of cool dampness beneath her back, a vague ache in her chest. She opened her eyes, the world an indistinct haze for several long moments before her vision came into focus. Luna looked towards the voice, and saw a familiar face. But... this is impossible. Regis Lucis Caelum stood before her, a smile pulling his features, tears shone in his eyes.
"Regis..." She mouthed, brows furrowed, voice lost. He reached a hand out to her, and she carefully reached for it and was pulled to her feet. The ache in her chest faded, the last of the blurs vanishing into detail. She was in her beloved sylleblossum fields. "Where... where are we?" She asked, looking around her. She tried to remember something, anything. And with that try, the memories finally came snapping back, Farewell, dear Noctis. A different ache returned to her chest, and she felt tears stinging her own eyes.
"Dearest Luna, you did well to..." Regis's voice caught, for the barest of moments, "to deliver the ring to Noctis," He told her, placing a firm hand on her shoulder, the other lifting her chin so he could meet the young woman's eyes. "Wayward, though my son may indeed be... he has made me proud. May you two, know happiness." He told her, a sad knowing smile on his face as a rare tear graced the man's face.
"I am happy, to know I fulfilled my duty as Oracle... and gave Noctis what he needs to do his part," Her voice cracked, and she strained to hold back a sob. If only... I could have touched him. Just once more... felt his arms, heard his voice. But her fight was over now. The fight now belonged to him. "Now... I just need to watch over him." I promised him. She knew his fate mirrored her own, that enjoying the peace and life he would restore to Eos was not to be gifted to him. That one day, he too would face the gates and join her. I'll wait for you.
"Oh, my dear..." Regis brushed his thumb across her cheek, smiling at her. "If only things could have been different. I wanted nothing more than for you two to have the life you dreamed. I may be old, but I am no fool," he chuckled, knowing about their special notebook and the black dog that seemed to grace the citadel from time to time that would mean Noctis's mood would be jovial for the next several days. The black dog had always been a harbinger of joy for his son, and he had known Luna was behind such joy. "You meant the world to my son," he told her, and Luna felt her resolve dissipate.
Noctis had been the only thing that kept her sane during her twelve years under the fist of the empire. Aside from the fulfillment she would feel at healing others, the notebook was her only source of joy. When Regis had entrusted her the Ring of Lucii, she promised to get it to Noctis. She begged Regis to follow her that day. She closed her eyes again, feeling the damnable tears wetting her cheeks. She wanted to tell him how she felt about his son, about how much Noctis meant to her as well despite vast distances of land and time.
"And I see the bond worked both ways, I see it in your eyes how much he meant to you, and it warms my heart to know my son could capture the heart of one so kind and pure." The warm baritone continued. "You've both... made me so proud." He embraced the young woman, letting her cry against his shoulder, "Your part is done, your fight is over. Rest now."