A/N: My chief source for the lore behind this AU is W. B. Yeats' Fairy and Folk Tales of the Irish Peasantry, which includes the poem from which the title comes, "The Stolen Child."
For the World's More Full of Weeping
By San Antonio Rose
She would never be quite sure what it was about John Winchester that caught her eye. He wasn't even Irish. As she watched him grow, she was keenly aware of his faults—his temper, his self-absorption, his tendency to be a lone wolf and to pursue his desires with a single-minded passion. But the closer he came to manhood, the more pleasing he became to the eye, and the more she realized that he had many good qualities as well. He began to come out of himself, to see that others might need his help and to give that help gladly.
Somehow she fell in love with him. But he fell in love with someone else.
Mary Campbell was a hunter, and when at last she returned John's love, a confrontation became impossible. There were rules about this sort of thing, and besides, pure common sense dictated that crossing a hunter for any reason would be most unwise. But for some reason the relationship seemed doomed from the start.
So Niamh of the Golden Hair bided her time while John went off to war, secretly slipping protective charms into his gear before he left. And when he returned, she watched in sorrow as he proposed to Mary.
But before Mary could make up her mind to accept, she and her parents left town on a hunt and did not return.
It took some doing to learn through the grapevine what had happened. A demon had surprised the Campbells on the hunt and had killed Samuel and Deanna to force Mary into a deal. And no sooner had Mary started back for Lawrence than a rogue angel had run her off the road, for reasons Niamh could neither learn nor guess.
It seemed crude to attempt to contact John then. But five years came and went, and he never learned of Mary's death, for she had been buried under the false identity she had used on the hunt. And finally, on a night of rain in April of 1978, Niamh could stand his pain and her own desire no longer. He dreamed of his Mary, whose form was not too different from her own... and she went to John's bed. Her gentle kisses woke him, and though he wept and demanded to know where she had been, it was none too hard to deflect his questions and convince him to save speech for another time.
She hadn't intended to stay for more than a few days, at least not at first... but in their play, she forgot herself, and sometime in the night, she felt a quickening in her womb.
She couldn't tell John, of course. He thought she was Mary, and a mortal woman couldn't know so soon that she was with child. And in all her long years, she had never before borne the child of one of her mortal lovers, not even of Oisin; she had no idea what the court would say if she returned in this state. She didn't know what to do.
So when John suggested the next morning that they elope, Niamh agreed without even setting a geis or any kind of spell upon him. She could stand to be his wife for twenty years, to see their son grown to manhood, and she would find a way to tell them the truth by the time she had to leave these mortal lands, perhaps even convince him to come back with her. And if John noticed that that she was not quite the same as his Mary or that she gave all his iron cookware to his friends Mike and Karen, he never mentioned it.
It was surprisingly easy to adopt the identity of Mary Campbell Winchester. And bearing a child was less difficult than she supposed. The hardest part was keeping her powers, and later Dean's, hidden from John and from their neighbors. But Dean was a delight, so much so that she wondered why she hadn't tried motherhood before. And after four years of marriage, she decided to try it again.
But no sooner was Sammy born than Niamh sensed a demon looking for him.
She warded the house in every way she knew. She searched for lore on demons, dared to seek out hunters who didn't know the Campbells personally for advice. She tracked down a gun said to kill anything, but Daniel Elkins had it in an iron safe, so she couldn't steal it. And the more she learned about this demon—Azazel—the more she feared that she wouldn't be able to drive it away on her own.
So on Samhain, she sent word to her father that she might soon return. Two nights later, when Sammy turned exactly six months old, Azazel fought past Niamh's wards and into Sammy's nursery. She was waiting for him, with Sammy clutched protectively to her breast... but a brief struggle was all it took for her to realize that her fears were correct. He was too strong for her.
One quick spell, and she was in Dean's room, shaking him awake. Another, and they were on her trusted steed, headed home.
Once her children were safe, Niamh had every intention of bringing John to Tír-na-n-Og. But her power was blocked by Azazel, and the sentry she sent to fetch John returned with the report that the house had burned down around him and that John, believing the three of them dead, was out of his mind with grief and would by no means heed the summons. Three times she sent for him; three times he called the herald crazy and would not come.
And the fourth time, the herald reported that John had become a hunter, bent on revenge.
The boys missed their father, of course, and Niamh was ill pleased that John had chosen such a path. But only a full mortal could remain sad for long in Tír-na-n-Og. Sam and Dean grew swiftly into fine, strong young men, clever and powerful and good, and they loved their mother and her people dearly.
It was Mananan, however, who finally put his finger on why Azazel had wanted Sam. At the time of Mary Campbell's deal, John Winchester had been the last of the line of men who could be the vessel of Michael, which meant that his second son could be tainted with demon blood and made the vessel of Lucifer. At least, that would have been the case if John had married Mary. But now both boys were half-Sidhe; it was not likely that either of them could be taken as angel vessels, said Mananan, given that the Sidhe themselves had fallen from Heaven but were not damned as demons were.
When two-and-twenty years had passed in mortal lands, word reached Tír-na-n-Og that Azazel was at work once more, seeking both a vessel for Lucifer and a chance for his own revenge on John for the loss of Sam. Sam had little memory of his father, but Dean leapt to his feet and declared his desire to find and aid John in any way he could. Sam would not let Dean go alone... and Niamh would not let them leave without her.
So on Samhain, the three of them made their way back to Lawrence, Kansas, and spoke with a mortal wise-woman named Missouri, whom John had contacted after the fire. She sent them to Bobby Singer, one of the hunters Niamh had sought out for demon lore, and to Ellen Harvelle, the widow of one of John's hunting friends. It was Ash, Ellen's foster-son, who first realized that Niamh was of the Sidhe, but he vowed not to tell anyone other than Ellen. Bobby likewise figured it out and promised to keep the secret. And when Ellen and Bobby and Ash finally found John for her, Niamh rewarded them richly.
It was in Chicago that they caught up with John, who had been lured into a trap and ambushed by daevas. Sam put the daevas to flight while Dean healed and freed John and Niamh banished the demoness who had trapped him. When the brief battle was over, John stood staring at them in disbelief.
Dean finally broke the silence with, "You're welcome."
"Who the hell are you?" John growled.
Niamh couldn't suppress a sigh. John had aged greatly, his hair and beard beginning to turn grey, and his sorrow and his hard life had taken their toll on both his body and his mind, making him not feeble, but hard and suspicious. "'Tis changed you are, my love," she whispered.
His gun was in his hand a second later, aimed at her heart. "Who the hell..."
The bullets were lead, not iron, but both boys jumped in front of her with a cry of "DAD!"
John's eyes went wide as he stared at Dean, color draining from his face, and his hand shook. "No... no, it can't be... my boys, my wife, they died..."
"Mary Campbell did die," Niamh replied. "But 'twas not she you wedded, but I, and that for love."
"Th-then who... what..."
"My name is Niamh, daughter of O'Donoghue, king of the Land of Youth."
"Land of—y-y-you're... you're a fairy?"
"Yes, John. I would have told you if I could, but there was never a good way before the fire, and after you would not heed."
"Why? Why did you leave me?"
"Our sons were in danger. With my people, they were out of Azazel's reach. I sent for you three times, my love, but you would not come."
A tear rolled down John's cheek. "Dean?"
"Yes, Dad, it's me," Dean replied. "We missed you."
Sam huffed a little. "Nobody's called me that in a long time, except Dean. It's Sam."
"Mary—N-Niamh... I've missed you so damn much..."
The boys stood aside, and Niamh embraced John and let him weep. And weep he did, the harder when his spirit recognized the bond of marriage that tied his soul to her.
"Come away with me, my love," she whispered.
"I can't," he replied. "Not now. Not 'til I've stopped this damned demon."
"Then we're staying," the boys chorused.
John released Niamh and shook his head. "No. No, boys, it's too dangerous."
Sam huffed. "Dad, in case you hadn't noticed, we're not exactly powerless. We can help you."
"Sam, Azazel's one of the rulers of Hell. You're only half fairy."
"The power of the Sidhe alone is not enough to defeat him," Niamh agreed. "But there is another way." And she told John about the Colt.
John swore. "I've been asking Elkins about that damned gun for six months."
"Well, he won't have it much longer, whether he gives it to you or not," Dean stated grimly. "I see a nest of vampires nearing Monument, and they have a grudge to settle with him."
Niamh chuckled. "Vampires we can handle. And I think I can assure you, John, that Daniel Elkins will give you whatever you ask of him once the vampires are dead."
He studied her face. "You're serious? You really want to help me? And... and you'll take me with you when we're done?"
"Yes, John. I still love you."
"Okay, then. Th-thank you."
They left Chicago then, John in his Impala and Niamh and the boys on their horses, and went to Colorado to rescue Daniel Elkins. It took only a minor spell on Niamh's part for Elkins' gratitude to extend to relinquishing the Colt. Sam and Ash traced Azazel quickly, and while Niamh and the boys held the demon trapped, John shot it through the heart. Sam set the remains ablaze with a wave of his hand.
John decided to leave the Colt and the car to Bobby. When he'd made that delivery, Niamh finally consented to kiss him, and the years fell from John until he looked nearly as young as Dean.
Bobby cursed under his breath in wonder. Then he asked, "What do you want me to tell people?"
"Just tell 'em I'm gone," John replied, not taking his eyes from Niamh's face. "No need to say more than that."
Then the four of them bade Bobby farewell and left his house.
"One thing I've been wanting to ask," John said as he mounted the horse Niamh had summoned for him. "I thought Oberon was the king of the fairies."
"Only in Scotland," Niamh laughed. "I'm Irish."
Dean looked over at Sam. "Race you back."
Sam just grinned and took off. Dean yelped and galloped after him. John and Niamh laughed and shook their heads.
Then Niamh put a hand on John's arm. "Come, my love," she whispered. "Let's go home."
John smiled. "Gladly."