A/N: Thanks for the feedback so far. Please, let me explain myself. I had this idea, even though it seemed strange, and I know Mutants are supposed to be sterile, but then I remembered where I had that idea from:
I played Fallout 2 (quite a while ago, but still), where you had a Super Mutant called Marcus as a party member. Likewise, in that game you had the opportunity to stand your party a round in a brothel in a city called New Reno. When afterwards the player opened the dialogue option with Marcus, the mutant expresses his hopes of not having gotten the girl pregnant. When questioned about the fact that mutants are supposed to be sterile, he answers along the lines of initially yes, but that it would "take a few years after being dipped to get the juices flowing again". So forgive me for not going into detailed research in Fallout Wikis or the like before writing my story. I did not see there would be a major problem in that department. And believe me, I am going to wrap all things up nice and clean. At least I try very much to do so. Maybe the story still is not more than random crap, if you think so, let me know, and I just delete it and we forget all about it. I meant no offence to anyone, anything or anywhatsoever.

It had been sheer chance that had brought her, bloated, swollen and bleeding, to Agatha's door. The old lady had taken her in without hesitation, but had it not been for another sheer chance, i.e. the fact that this very night the trader caravan led by Doc Hoff was camped at Agatha's to travel onward the next morning, she would have died anyway.
As it was, the Doc didn't have his name for nothing, whatever else he was now, he did have medical training. Without him, she would have bled to death.
And between the two of them, Agatha, who had born five children herself, and the Doc, she managed to survive a second hell of blood and pain.
But Cassandra knew that it had been neither his nor Agatha's skills that really had made the real difference, the difference between life and death for her. It had been her decision that she was doing this for him. He was dead, and all she could do now for him was to take care of the last thing that remained of him here on earth.

And yet, when she had looked at her son's face for the first time, she realized that she was not only doing it for his father's sake, but also for his own. He had not asked to be born. He had not chosen to be sired and conceived of a love so unconventional. The Doc told her not to attach herself too strongly to him, as he didn't have a high chance of survival. By rights, Doc Hoff had said, he shouldn't be here at all. Mutants were supposed to be sterile, and even hadn't they been, it was thought impossible that they were genetically compatible with humans any longer. Which meant any offspring of a mutant and a human would be unviable.

Cassandra looked at the sleeping face of her son and blinked her tears away. There he was, almost three months old now, and still alive. True, he had almost killed her, being born. But it had not been his choice. It had not been his fault. But what would become of him now? She had promised him, the day he was born, that she would take care of him, protect him as his father had protected her. But that didn't seem very likely now, as she never had made it out of bed after his birth. It was the loss of blood first that had weakened her, and even after these three months she still had no strength back. He was draining her bloodless, and still, he was never able to eat his fill. That was why she was constantly hungry and thirsty, but without any more medical help, she knew, she would perish. Her body, drained by the pregnancy in grief and malnutrition, what with her aimlessly wandering through the wastes, and the enormous loss of blood in giving birth, had used up all its resources.

Therefore had she finally given in and allowed Agatha to send for help via a caravan. The old lady could not support the boy, without her, his mother, he would die as well. And she had promised him to take care of him, as she had silently promised his dead father the same.

She would have preferred never to see anyone of the Brotherhood of Steel again. Never to see the Citadel again, where his grave would be. But to fulfill the promise to her son, she needed the help she could only get there. Only the medicinal facilities of the Citadel had the means to save her, that much she knew. If they could help her son to survive, a being of mismatched genetics that had no biological right to exist, was another matter entirely. Only time could tell that.
But he looked healthy enough, she always thought. He had two legs, two arms, ten fingers and ten toes, two eyes, a nose and a silky shock of black hair.

It was only this moment, as she toyed with his forelock, that she realized the implication if his hair. She herself was red-haired, as her mother had been. So this color of hair must have come from his father, something that had survived the mutation somewhere in the genetic code, even though Fawkes himself had been completely hairless. And at that moment she realized that her son was not only her promise to a future, but also a link to a long bygone past. Most likely, he would resemble his father, his father as his looks had been original genetically coded. In twenty or twenty-five years or so she would look at him and, to an extent, look back across the years into the face of the man she had loved; his face as it had been before the mutation had so irrevocably changed him.

This realization made her shudder, and at the same time brought new tears to her eyes. The pain would simply never end. Not before she had taken her last breath.

But at that moment the door opened and Agatha poked her head in, tearing her out of her painful musings. "Cassandra? Dear? I see a caravan coming."
She felt relief and dread simultaneously. If only she could forget about the past… but the sleeping form of her son beside her reminded her of the fact that she could never forget, not as long as he lived, and most likely not after, either. So she might as well try and pull herself together to make the best of her life, for his sake. And if that meant getting back in touch with people from that past, along with all the painful memories it would bring, then so be it.

It was not long after Agatha had announced the caravan that Cassandra was awoken from a nap she had dozed off into by a knock on the door. Sleepily she lifted her head as the door opened, and despite herself, she had to smile as she saw the familiar face.
"Hello Sarah."
"Cassandra." Sarah Lyons shook her head and swallowed. "Good god, girl, what possessed you to wander off like that, letting everyone believe you're dead?" She swept her eyes over Cassandra's haggard form. "Not that you don't look more than half dead already…"
Cass looked down again and avoided her eyes. "I couldn't stand it."
"What? Being cared for? Having friends?"
"No." She swallowed her tears and managed to look into her old friend's face again. "I couldn't bear to look at his body. I just couldn't. I couldn't bear to be near his grave. I'm sorry, Sarah, I just couldn't do it."
Slowly, Sarah walked up to her bed and sat down beside her. "His body. His grave."
Cassandra avoided her eyes.

"Cass…" With a deep sigh, Sarah ran her hands across her face. "Cass… there is no grave."
Feeling cold, Cassandra suppressed a shudder. "So? Yes, I already supposed you had to burn the behemoth to avoid it poisoning the whole area for months to come with rot gas. It makes no difference to me."
"It should." Sarah dropped her hands and stared at the ceiling for a second before moving her gaze back to Cass. "Yes, we burned the body of the behemoth. But we had to cut it into pieces and move it away from the Citadel; to be sure the fire wouldn't spread to our compounds."
The silence that followed made goose bumps spread on Cassandra's whole body. She found it very hard to look at Sarah Lyons again.
Her friend returned the look with a face carved in hard lines of sorrow. "Cass…" Then she took a deep breath. "He wasn't killed that day."
The world spun, and Cass found herself suddenly steadied by Sarah's arms. "Cass? By god, you're weaker than I thought. We need to get you back home, and fast."
"Home…" but her vision was wavering, and the world was still swaying. "But…"
"It is as I said, Cass. I don't know how many guardian angels he had or if he had nine lives like a cat. If so he used at least six or seven up that day in a go. He was severely injured, and by the time we found him his wounds had gone septic, but for some reason, he survived. He is a strong man, Fawkes."

Cass was slowly coming to her senses again, so she didn't fail to notice the strange tone Sarah had used in her last sentence. She tilted her head, wondering if she had been mistaken. Was it displeasure, or had she imagined it?
Sarah took another breath. "He… told us… about you. What had been between you and him. It was… a bit hard to swallow at first, but we knew him, and after all, it had been him saving us all in killing that behemoth and almost sacrificing himself with it. It was… very hard for him when you had vanished. I think it was only the thought of you that made him hang on and survive. When he finally came round, and realized you were gone…" She broke off and looked away.
Through the shock, Cassandra felt hot tears burn in her throat. "I've abandoned him…", she croaked, hoarse with tears. "God, I've abandoned him. But honestly, Sarah, I thought he was dead! Please! Had I only thought there was the slightest chance of him being able to survive this, don't you think I'd have stayed?"
"You probably would", her friend gave back, placing a hand on her trembling shoulder. "I know you would. But this… it was… when the caravan brought the news about you, and what happened to you…" She looked away again. "He went into his cell and smashed all the furniture into shards. Then he was gone for a few days, and I have no idea what he did, nor where he was. He came back just as we were ready to set off, and he came with us. I must say, no one, me included, dared to ask him where he had been. It is as if he has turned to stone."
"But…" Her voice and her thoughts failed Cassandra simultaneously. "But I…"
Yet her friend said nothing.

Cass could only watch helplessly as Sarah got up again and went for the door. Her thoughts began to run wild in panic, and her wits were scattered like a hoard of spooked chickens. What was the meaning of this? He was alive… and yet. There was something lurking under Sarah's words. Something that frightened her. Something that did not allow her to feel the elation she felt she should have been feeling.

And then Sarah returned, and stood wordlessly aside after entering, followed by another knight in full power armor. It took Cassandra a few moments to realize that this knight was much bigger than Sarah Lyons, so large, in fact, that not only had he to bend down to fit under the frame but also strafe sideways as his armor was so broad in the shoulders he didn't fit through the door.
Her heart stopped and took a few seconds to start beating again. Unable to move, or say a word, she stared as the knight moved his hands up to his shoulders to remove his helmet.

Stone. It was an apt description Sarah had used, all right. His face was stony, unmoving as if carved from granite. Two ugly, whitish scars ran down the right side of his face, one of them right through the eye socket that now was empty. The other eye stared down at her with an expression she could not identity and that made her afraid, as afraid of him as she never before had been.
She could not hold that gaze and looked away. What had she done? They should be falling into each other's arms right now, shedding tears of joy. What was happening?

"Fawkes…", she began helplessly. "Fawkes, I'm sorry… I thought you were dead… I really did!" She managed to look up again, but his expression had not changed. "I'm sorry. Please, forgive me… Had I thought there'd be a chance I'd have stayed. Honestly! Please… please believe me…"
A deep sigh heaved his ribcage and shoulders, still it took a while before he answered, his voice devoid of any emotion. "I believe you. I can understand why you went. I do." Then he took a slow step towards the bed, and another one, so he was standing beside her. "But this… Cassandra… why do this to me?"
Hearing his voice again after believing him to be dead for so long a time should have filled her with joy. But looking up at him, Cass could only blink in utter confusion. "What… what are you talking about…?
"That!" And he pointed at the baby beside her.
Cass flinched at his sudden movement and hastily pressed the baby to her breast. "What…?" She swallowed, looked at the face of her son, looking crossly at her for grabbing him like this, and back at the man standing beside her. As if he needed protecting from him… or did he? She was confused, afraid, and completely at a loss as to why there was so much hostility in his voice. "What… tell me… what is wrong with you?"
"What is wrong with me?" His nostrils flared. "What is wrong with me?", he almost roared. "I lie for months at the threshold of death, surviving only because I kept thinking of you, then learning that you have disappeared from the face of the earth, only to learn months later that you have been found, needing help because of…" He gritted his teeth. "Because of this." He took a deep breath. "And after spending months wishing I could find you alive, I suddenly began wishing you were really dead! And you ask me what is wrong with me?"
He looked down at her and squarely into her face. "Tell me, Cassandra. Please tell me the truth. Is this a child of rape?"

Cassandra blinked as if he had slapped her, and suddenly, all at once, everything fell into place and she realized what the reason for his hostility and Sarah's unease was.
"Rape?" she swallowed, and suddenly felt cold with fury and desperation both. "You realize… I mean you know that I almost died, giving birth to him?"
"I do, and I know it is not his fault…" he began, but Cassandra cut him off short.
"Yes, you know. And you think you know it all? Did you think I just…" She shook her head and felt as if she had been kicked in the stomach. Fury and the feeling of utter betrayal and disappointment lent her a strength she had not known she possessed.
"You fucking, bloody idiot! Was that what you've been thinking? What you all have been thinking? That I ran away after thinking you dead I'd have nothing better to do than get into the pants of the first man I meet? Shag myself up and down the ramps of Megaton? That what you've been thinking?"
Tears of fury and frustration streamed down her cheeks by now, and she could do nothing but scream at him. "Asshole! And wishing me dead for it! The gall! Then why did even you come here? To insult me and drop me like a hot coal?" She wiped her hands across her face, but the tears wouldn't stop. "Preferring me to be raped, no doubt, because then it wouldn't have been my fault! Bastard! Did you think I would have gone through what I did for a child of rape?"
"Cass…", he began, but Cassandra was simply unable to stop.
"And to think I've missed you so much that death seemed the easier way out! And I only did it for you! For you! Can you count, Fawkes? Can you count?" She gulped for air, her tears still hadn't stopped, and she could not stop herself, either. "He's three months old in a week! Plus nine months pregnancy makes twelve! He was conceived a year ago, give or take a couple of weeks!" She was almost hoarse by now, and the baby, uncomfortable and afraid by her screaming, was mewling in her arms. She ignored him.
"And how long ago was that fight, heh? How long ago? Answer me!"

His face had turned even more stony during the last few moments, if that was even possible. And he swallowed, cleared his throat several times, and still his voice wouldn't carry, it broke into a hoarse whisper as he answered. "I…"
"Eight months ago." Sarah's voice was flat and cold as she answered in his stead.
Her energy suddenly spent, her fury vanished as suddenly as it had appeared, Cassandra sunk back into her pillows and closed her eyes. Hoarse from screaming, and utterly drained by her outbreak, she could only whisper. "But you were rather thinking I should have been raped instead of just bloody counting the months."
Fawkes failed to answer. As if he had turned to stone completely.
"He's yours. Your son." The tears flowed again, but silently this time. "For the sake of your memory did I survive giving birth to him. For that alone."

The silence in the little hut was so heavy it was tangible. No one moved, until finally Agatha took the initiative by taking the crying baby out of Cassandra's unresisting arms. Fawkes made a step up to her and Agatha stopped, letting him take a long look at the child, standing as silent and stone-faced as a statue. She didn't hold out the boy to him, nor did he reach for him, and with a nod to Sarah, she then left, and the paladin followed her outside, silently closing the door behind her.

Fawkes mutely returned to her bed, and Cass turned her head to look at him. She felt a gulf between them as deep and as broad as heaven and earth together, and wondered if they would ever be able to reconcile. Then his empty stare focused and found her face, but before she could say anything, he slowly blinked, and a single tear trickled down his left cheek.