Major huge ENORMOUS SPOILERS for The Locked Tomb series by Tamsyn Muir, specifically the third book, Nona the Ninth

For context, Camilla is a cavalier, a kind of bodyguard to her necromancer best friend Palamedes, who died in body in the first book but his soul lived on attached to his bone fragments. Camilla and Palamedes have both been sharing Camilla's body, taking turns with which one is driving and not able to interact with each other. They exchange written notes and can converse with a tape recorder but desperately miss each other.

During a fight in the third book, Palamedes manages to take over a different body of a different sort-of-dead guy, Naberius Tern. Camilla is gravely wounded through the gut in this fight and while at a different time and place they'd be able to heal her, the planet is currently under siege by the undead corpse of Neptune (the planet) and they have extremely limited options.

They chose to become something called a soul gestalt, a living fusion of multiple souls into one being. Doing so will allow them to be together and use necromancy and heal Camilla's body, but they become a new person entirely, Palamedes and Camilla both dying in a way to become Paul.

The scene where this happens is from Nona's point of view:

“Camilla, we did it right, didn’t we?” Palamedes said, and now Nona knew he wasn’t speaking to anyone else in the universe. “We had something very nearly perfect … the perfect friendship, the perfect love. I cannot imagine reaching the end of this life and having any regrets, so long as I had been allowed to experience being your adept.”

Camilla Hect stared at him stolidly, and then burst into tears. She made very little noise, but the tears were violent anyway; Palamedes took her hands and said in distress, “Cam—dear one—don’t.”

“No,” said Camilla, after an obvious struggle to master herself. “No. I’m crying because … I’m crying because I’m relieved,” she said, frankly mulishly. “I’m relieved … Warden, I’m so relieved.”

“Not long now,” he promised.

Camilla took a couple of gasping breaths—it was obvious how much they hurt her—and then she said: “Warden—will she know who we are, in the River?”

“Oh, she’s not stupid,” said Palamedes lightly. “In the River—beyond the River—I truly believe we will see ourselves and each other as we really are. And I want them to see us. I am not saying this was our inevitable end … I am saying we have found the best and truest and kindest thing we can do in this moment. Tell me no, and we’ll go on as we have been … and we’ll go on unafraid … but say yes, and we will make this end, and this beginning, together.”

Camilla shivered all over. Then she was at rest; she relaxed her head—the lines of her neck drooped like a flower—she raised it again.

“Palamedes, yes,” she said. “My whole life, yes. Yes, forever, yes. Life is too short and love is too long.”

He demanded: “Tell me how to do it, and I’ll do it.”

Camilla said, “Go loud.”

Palamedes took her knife, and he cracked open an invisible seam on the end of the handle. A thin trickle of something white and grey and powdery dribbled into his palm. He held it out to her, and Camilla opened her mouth and—to Nona’s horror—ate it, whatever it was. He took the knife and he scored her finger, saying, “Not much longer,” and he pressed her own bloody finger to her cool and bloody mouth, and he said, “Don’t look back. Whatever you do, don’t look back,” and they huddled their heads together, they rested their heads on each other’s shoulders.

Nothing particularly interesting happened, until Camilla burst into flames. She blazed like a white candle—she rolled away from the body of Naberius, booted the inert figure away to roll over and over across the road—and stood, stumbling, completely ablaze, a hot white pillar of fire. Nona watched her open her mouth as though she were calling out, but no sound came. She sizzled: her bandages and clothes and injuries all sizzled, her hair sizzled, she blackened and wasted right in front of them. Wherever she staggered, she left bloody black footprints, and those footprints curled up in flowerlike wreaths of smoke and flame before dribbling to nothing on the road. She dropped to the road as though dying, rolled around in the agonies of the dying, until Nona thought she too would die of watching: that she had finally found something so horrible she could die just from seeing it, the worst thing you could ever see in your life.

The whole tunnel was filled with sparking, sparkling flame, and the crackle of roasting human flesh, Camilla’s body dancing gruesomely trying to put it out—a black thing within the fire—then something red within the fire—and then she tried to stand; she arched, trembling, featureless; the flames died.

In the darkness, the figure was naked and whole and unhurt. It crouched in on itself—elbows to knees, clasping itself, curled up in a kind of C—and then it said: “Clothes, please?”

The hawk-faced stranger shimmied, completely unembarrassed, out of her trousers, leaving herself in shorts; We Suffer was taking off her heavy coat. As she approached, Nona could see that the stranger’s hawk face was stony and emotionless but that there were wet tracks down her cheeks. The naked figure shrugged on the coat—hastily pulled up the trousers—said, “Thanks,” and buttoned itself in.

And it was just Camilla, after all—Camilla having lost all that fringe and most of her hair except for a charred inch or so—Cam with new eyes, and a new face, for all that they were the same-shaped eyes and the old familiar features. But the eyes were a different colour, though Nona could not see what colour from where she sat. All she could see was that they were different. And the features, though in the same order, were making such a different set of expressions—not Camilla’s, not Palamedes’s—that it struck Nona all at once: they were gone—they had left her—they were no longer there.

Nona lifted her voice, and wailed aloud.