This story is set immediately after the duel with Palpatine in TROS, but in my version, Ben has just barely survived.
Ben's eyes open. He's damp, water trickling down his face, stinging his bleary eyes. He's shaking, wracked with an exhaustion beyond the body, as if the flame of his life force had shrunk down to almost nothing. A small red glimmer of lifeforce, flickering below. Below.
Below was Rey, in his lap. She was asleep. Right? His eyes widened in panic, just as a massive shiver ran through the quiet body he held. Ben relaxed to know she was alive, her weight keeping in their minimal warmth. Warmth. Aside from this tiny beacon of human heat, all was water and dim grey light.
Ben's leg awoke and suddenly a spasm ran through his knee down into his ankle, which, he could not help but notice, was not properly aligned anymore. Hissing in pain, Ben became aware of a few more injuries: his head throbbed along unseen muscle connections running down into his neck. From there, his back spasmed. The flame inside shook and wavered. Rey's eyes flew open and she sat up, gripping his arm.
Her eyes were bloodshot, strained, as they locked onto his. Panic. He saw into them: Palpatine casting Ben off a cliff; Rey left to deal with him alone. The voices of the Jedi transmitting their strength through what felt like glimmering cosmic rivulets, coursing through her mind and body and through her lightsaber to defeat Palpatine. Then pure white light and booming silence, followed by blackness. In her memory, his face swam into focus. He had never seen his own face this way, full of wonderment and openness. Then in her memory, Rey kissed him.
A violent twitch from Rey brought him back to the real world, and they both panted slightly from the effort of transmitting memories in their decimated state. She was damp all over and so was he. He was backed up into a wide nook in the back of a cave, barely cradled by wet rock. His ankle throbbed again, so hard his chest clamped and his vision dimmed. Rey shivered and her teeth chattered. Ben drew her in but his shoulder and neck were smashed from his long fall. The darkness closed in and he fell down a long chasm, pulling up briefly before tipping back in end over end, over and over. Rey chattered, hummed and vibrated. Their flames flickered silently as the downpour blew outside all around them. Each of them was haunted by memories of the previous hours, days, weeks, years, an inky damp darkness enveloping them from all sides.
Rey crept along the dark passages of Exegol, hunted by a vague blackness set on finding and destroying her. She had no lightsaber and the force was not with her.
The blackness had form, a multiplicity; it could walk, dart, hide - the black forms pursued her relentlessly down the claustrophobic passages. They were always a half step behind her no matter how desperately she scrabbled around corners and over outcroppings, hoping against hope she could stay just out of reach. Rey felt a tell-tale emptiness at her hip where her lightsaber should have been. She reached out in her memory to this object but aghast, watched as it was sucked down into murky black sand and was gone.
Her stomach dropped and the shimmering rivulets of energy which connected her to the galaxy dissolved into ash in an instant.
From behind a grey stone outcrop swerved a powerful dark figure and her miserable reverie was broken. This figure was their leader, the driving force behind her treacherous pursuants. Fear surged in Rey as the figure ignited its cracking red lightsaber and closed the gap between them into two long strides, raising the red sword above its head with the speed of a snake about to strike. Rey looked up as the figure brought the blade crashing down around her like a wave, drowning her in blackness and sending her spiraling through nothingness.
A sound broke through the ink; it was her own voice. She could feel it vibrating in her mouth and chest. She was shouting and scrambling, scrambling across wet stone until she tipped over into a deep puddle and was halted, halfway submerged in the cold grey water.
Ben yelped as he scraped across the cave floor, dragging his bum leg behind him. With his long arms, he crawled quickly and grasped Rey around the waist to scoop her out onto higher ground.
Water drained from her onto the floor. Looking down, Rey saw a pair of cold blue hands, her hands, damp inside and out like the rest of her. Involuntarily her lip quivered, then her whole jaw, and she choked on emotion, moisture rising to her eyes and streaming down her face. Wails, moans escaped her throat in sequence, mixing with the crashing wind and watery downpour outside. Her streaming tears made no difference to her current soggy state. She cried and cried, unable to stop herself.
Ben's face shuddered and twisted as if his features were about to explode. He gasped and barked and tears streamed from his bloodshot eyes too. Darkness smothered the edges of his vision and the injured arm on which he leaned buckled. He crashed onto his elbow, sending shooting pain up through his shoulder. He tipped over and lay back on the ground, eyes open and streaming freely, water running down his face to the wet floor.
Rey collapsed on top of him, rocking in the shudders of his chest, adding her own shudders and tears to the pile. His flesh was soft and warm. Sleep came fast, as it does after a good cry, and she was washed away into unconsciousness. She dreamed again of a fiery sword; still this time, and upright. Although the sword was distant and flickered like a candlewick, as she sat with the sword its light centered her. Sounds abated. The sword flickered dimly. Did the light increase? The object emitted a low crackle and Rey's heartbeats slowed as she felt its small warm glow on her face.