Disclaimer: I clearly don't own anything related to Buffy the Vampire Slayer or How I Met Your Mother. If I did, I'd be writing canon rather than messing with it.
Buffy: Cannon through Chosen
How I Met Your Mother will be canon apart from the things that are changed in the story. If an episode or story arc is skipped, feel free to assume that it happened the same as in the show and I just have nothing to add or twist to it.
"A people without knowledge of their past is like a tree without roots" - Marcus Garvey
Prologue
Willow stumbled out of Robin's office to get on the bus out of town. Her part of this fight was over, and she was exhausted. Weaving as she walked, she turned every way she could, eager to escape danger in her defenseless state. The clamor of battle raged around her as she leaned against the wall, resting for a minute before taking her final steps outside into the protective sunlight.
This was a mistake.
With a violent shove she found herself being pushed forward onto the ground. Willow's sight came back into sharp relief, adrenaline surging through her, fueling her movements. She rolled forward, coming up into a crouch. Taking a moment to privately congratulate herself on executing a Buffy-esque move, she spun quickly up on her heels, bringing the backup stake she kept in her waistband up in front of her. She stopped short, taking in the view before her. Where a moment ago there had been a clean (for a high school in the middle of an ultimate battle between good and evil that is) hallway, there was now walls painted with the red blood of someone. Two Turok-Han were prowling back and forth, trying to circle a smaller, female figure with a stake in one hand, and her other clutching her neck, blood running between her fingers.
"Go Willow!" came the sharp voice, though some of it's harshness was lost in the speaking, as if every moment was taking more and more strength from it.
Willow gasped sharply, her body forcing her to quickly gather the air she needed to reply before it was too late.
"Kennedy! Are you ok? What happened?" Willow stepped forward to reach out to her, but at that moment, the Turok-Han attacked. Kennedy with a swift leg sweep brought down the first, and drove the stake into it's heart quickly, but her injuries, the full extent of which Willow couldn't tell, slowed her down enough that the second vamp got a chance it never would have any other day. With blinding speed and the violence borne of a creature who likes nothing better than bloodshed, the monster drove it's clawed hand straight through Kennedy's back.
A blast of magic reverberated through the hall, as Willow blasted the second creature into dust, but at this moment the exhaustion caught up with her, and before she could use this sudden second wind, she collapsed next to Kennedy. Teetering on the brink of consciousness, she could barely hear her lover's whisper.
"Will, please get up. Please get out of here. I had to come back for you, and I'm glad I did. I've known for a while that I wouldn't survive this, but you have to." With a great deal of effort, Kennedy reached out her arm, and shoved Willow awake, breathing her last.
Barely able to see through the haze and blood and tears, Willow looked into the now blank face of this young woman with whom she had shared so much. Kennedy was the reason she was able to do magic again. Kennedy was her rock, her support. Her assertiveness had pushed Willow into living her life again after Tara's death, and though the relationship was new, Willow had again lost someone she loved, someone she should have been able to save.
Barely able to breathe past the unreleased sobs in her throat, Willow crawled out into the sunlight. Finally out of danger of the battle, she let herself go, her screams of emotional pain mixing with the chanting she was performing almost unconsciously. With a flash of light, Willow disappeared, not even able to hear the small thunderclap created when the air rushed to fill the void she left.
Willow awoke with a start, but sitting up was the wrong choice. Halfway into the air, she let out a tremulous groan and flopped back down into the bed. Keeping her eyes closed she began to review what she knew. It had been nearly a week since she had magically transported herself out of Sunnydale and into the New York suburban home of her grandparents. Unsure where to go at the time, her chants had simply asked to get as far away from that place of pain as she could, and as best she could figure, her subconscious had directed her to a place she knew well. Her grandparents had of course taken her in and given her a room. Her parents may have been distant her whole life, but there was still love there, and her grandparents had certainly never been withdrawn.
Of course, the downside of their concern for her meant she could hear whispers late at night, wondering what could have happened to her. She knew they planned to ask where she had come from, and what had put her in such a state, but she had worked out a plan.
Willow wasn't the best liar, but after spending so much time with the Scoobies, she had picked up a few things. Besides, given a week to plan, with no physical capability to do anything but lie in bed and think, she was able to come up with something that was fairly airtight.
It should have been easier though. Willow had realized after her first night there that a combination of activating the slayers, killing those Uber-vamps and the transporting herself across the country had left her drained of any ability to use magic. It was slowing her healing as well, the bruises and injuries from the battle only just now starting to feel better.
She had kept an ear open for her grandparents as she sat up again, but more slowly this time, and was pleasantly surprised to find that she could. She forced herself downstairs and decided that a good offence would be her best defence.
"Grandma, Grandpa?" she called into the kitchen.
"Willow dear, what are you doing out of bed?" her grandmother admonished
"I'm feeling better today. Listen can we go into the kitchen and talk? I have some things to tell you." Willow took a few steps towards the door before allowing her Grandmother to take her arm and lead her the rest of the way.
"What's on your mind?" her grandfather joined them in the room, taking Willow's other arm and helping her into a seat.
"I just wanted to thank you for taking such good care of me, and give you some explanation for why I'm here." Willow sank into a chair and absentmindedly grabbed a small roll, toying with it while trying to seem as if she was having trouble starting.
Finally she broke the silence, and then told her grandparents the best story she could, mixing fact with fiction, and being unafraid to make herself look culpable as well as her inventions.
The story she told them was one of half truths. She spoke of a relationship that sent her into a bad place during college, one of addiction and an inability to control herself. After getting help, she returned to college only to find a new relationship, one who she relied on too heavily in getting over that addiction. This fake boyfriend, whom Willow named Caleb (because why not give others a reason to hate that name as well) took advantage of Willow's dependance.
As she got to this part of the story, Willow put some effort into remaining vague, as if she were unable to talk too much about the difficulties he put her through. Finally, after a huge fight, she told her grandparents, she left him only for him to chase her, and force more drugs into her system. Angry at his betrayal and afraid or becoming addicted again, she ran as far as she could from home, and to a place she remembered only with fond memories. She escaped him there, but exhausted and feeling the effects of a new withdrawal, she was able to do nothing but rest the past few days.
They "oohed" and "ahhed" in the right places, and put comforting hands on her shoulders, and Willow felt guilty about deceiving them, but she needed them to believe this danger if the next part of her plan was to work.
The next few days, Willow spoke with her parents for the first time in years, They had moved away from Sunnydale, and what had started as a test to see if they would contact her first, turned into a long silence, only now broken because she needed to inform them of the next part of her plan.
Willow Rosenberg was no more. Instead, legally, Willow changed her name in order to escape her past, even if it was a different one than her family believed, she still needed to run. Determined to get her life back to normal, or at least to what it might have been had she grown up anywhere but on a Hellmouth, Willow enrolled in college at Wesleyan with new black hair, a new attitude and a new name. She left her grandparents house as Willow Rosenberg for the last time, and arrived in Connecticut as Lily Aldrin.