-Author's Note-

So I guess fanfiction is the cool thing to do, now? Either way, this is what came dripping out of my brain, and I really hope you enjoy reading it! I know crossover works aren't exactly as popular or well-read as cannon fics, but this is just what I wanted to write.

My thanks to Pendleton Ward and Joss Whedon, who put together such nice sandboxes to play in.

Bio Of A Nihilistic Prankster

Fandoms: Buffy The Vampire Slayer / Adventure Time

-Chapter One: Good Glob, I'm A Monster!-

-Willow Rosenberg's Basement-

Cinema Gone Wrong was a long-enduring tradition. Jessie had started it, thoroughly convinced that a terrible movie could be every bit as enjoyable as a good one, and with all the added attraction of being half-price off the racks of Sunnydale's Video Emporium. Willow, Xander and Jessie, three teens on a budget, considered 'cheap' to be an essential quality for any Saturday night's worth of entertainment.

Jessie was gone now, but like many fine traditions, Cinema Gone Wrong had outlived its creator.

"Six String Samurai!" squealed Willow. Xander looked over from his spot on the couch and tried to restrain his laughter. Reason being, that-

"We've held off the apocalypse," he noted from his seat. "I really don't get why you make with the obsession for post-end-of-the-world movies." As he spoke, Willow shimmied backward and out of the crawl space storage with a boxful of VHS tapes. She glanced back, and a hurt look flashed across her face.

"But- Xander! Buddy Holly fights his way across the wartorn wasteland to become the rock king of a dystopian America! You love this one. You tried to make a katana in fifth grade-"

"And got suspended for it," agreed Xander cheerfully. "It was an important stage in my life." He frowned, and considered his half-eaten twinkie carefully. "It's just, you know all of these last-minute saves and apocalyptic threats-"

"Yes?"

"The changes to our basic worldview and revelations of things deeper and darker than have any right to exist?"

"Yes?!" He sighed.

"I'm disillusioned, Wills. My times of childish faith are gone. The world I knew has become a thin veneer. I'm overcome with sorrow, emotion-wise," he declared.

"Oh, Xander-"

"Which is why I just can't consider these movies realistic, anymore."

Willow paused, and glanced down at the pile of titles in her hands. It had such classics as "Attack of the Killer Tomatoes", "The Super Mario Brothers", and "Mystery Science Theater, A Reflection".

"Um, Xander, I'm not entirely sure you're supposed to take these seriously-"

"Suspension of disbelief has failed me! The same with television shows. I'm watching primetime television, and all I can think is 'why haven't these idiots been eaten?' or 'walking around like that? Joey and Chandler would have been abducted and sacrificed by demons!'"

Willow frowned, and wondered why her oldest friend considered this an existential crisis on par with opening the Hellmouth. Also, why he was watching Friends in any case.

"So, like with Six String Samurai, I just can't help but wonder, where the hell are the vampires? With the world's governments all collapsed, they don't need to hide. With most of the human race wiped out, they'd end up starving to death anyway!"

"I can see you've put a lot of thought into this, but I'm pretty sure it's exactly the wrong kind of thought."

"You just don't take me seriously." Willow grinned at that.

"Nope. It's just a story, Xander. Just... pretend the vampires turned vegetarian, or something. To survive."

"...Vegetarian."

"To survive!" Xander just shook his head in blatant wonder.

"Just... just pop it in." They settled in for the night of bad movies and artery-killing, and pretended there was still a third teenage body to occupy the floor in front and steal popcorn.

-Two Months Later-

Ethan's Costumes was full of teenagers trying to get in last-minute shopping. The racks were being scoured and emptied at a rate that delighted the owner of the store in question, and Xander Harris was carefully edging around the store toward the makeup and prosthetics. It was a relatively quiet aisle, since most of the costumes were packaged complete, including makeup and extras. 'Mix-and-match' was a much less popular option, for the majority of Sunnydale's lazy adolescents.

"And you want... Spock ears, young man?" the older British gentleman asked, amusement etched across his features. "I didn't think Vulcans were so popular amongst the younger generation..."

"I am not going as a Vulcan, or an elf, or whatever. I just need generic pointy ears. It's for my costume, to set it apart, a bit."

"And you're going as...?" prompted the proprietor.

"That is a secret. I'm keeping it from my friends as a surprise," admitted Xander. Ethan's eyes sparkled.

"I do love surprises... a spark of chaos does the heart good. But you're sure you couldn't let me in on the secret?" Xander shook his head regretfully.

"The walls have ears, my good sir. Observe." His hand thrust out, into a rack of costumes. There was a loud squeak, and Willow's head poked out from between two, identical witches' dresses.

"You can't keep it a secret forever, mister!" she gasped, oddly out of breath.

"Just until we leave the Buffster's house tonight, bud. Then all will be made clear." He frowned and leaned forward. "You're not getting sick, are you? You look flushed, and I really don't think Snyder will take an excuse for one of us backing out of trick-or-treating patrol tonight..." Willow's head shook.

"No, no, not sick. Could you, ah, let go?" Xander froze, and looked down. His arm disappeared into the rack of costumes below Willow's head, but billows of fabric hid where, exactly, it ended.

"I'm not grabbing your shoulder, am I?" he asked in resignation.

"No!"

"Oh. Let's just... repress this memory forever?" Willow just blushed harder.

"Gah!" He leapt back and shook his hand like it were on fire.

Ethan laughed. He was really looking forward to seeing how tonight panned out.

-Buffy's House, That Night-

Willow, Buffy, and Joyce took in the teenage boy with visible confusion. He shrugged at them and grinned, incidentally shifting the fake guitar prop and basketful of apples. Half of the fruits were painted white.

"Alright, I give up," admitted Buffy. "Kurt Cobain impersonating Johnny Appleseed?" That, at least, got a small chuckle out of Joyce Summers. Xander's grin widened until the fake fangs in his mouth flashed visibly. Buffy stiffened at the sight of them like a good little Slayer, but Willow squeaked and moved in closer.

"Oh my gosh, you really did it!" The sheet-clad girl seemed to suffer a small, involuntary nerdgasm.

"Did what? Xander?" prompted the older Summers woman.

"Not tonight, Mrs. Summers," corrected Xander. "Tonight I'm Marshall Lee, a vegetarian vampire fighting and rocking his way across an apocalyptic wasteland."

Buffy's eyeroll was so blatant it should have had a sound effect.

"A vegetarian vampire?" Xander shrugged helplessly.

"Pickings got slim once all the humans mutated into fish and sentient gumdrops," he said, pulling excuses out of his proverbial hat.

-Later, In The Chaos Of Ethan's Spell-

"Can't bash monsters, have to listen to some half-baked mutant vamp who can't even fly, and taking orders from a hot chick who's too friggin' intangible to put out," summarized Marshall Lee. A quick swing of his guitar knocked back another one of theose half-vamps who might actually be some dumb kid. "This night sucks. You all suck, and I-"

The vampire king cut himself off, and glanced off into the sky, to where it was just visible through one of the higher-up windows. He floated further into the warehouse, and completely ignored the fighting below. The bass guitar was re-settled over his shoulder as he carefully scrutinized the nighttime atmosphere.

His senses were going crazy. He could feel the collapsing of a great magical spell. Probably the one that had turned this holiday all kinds of nuts. Outside the window, pint-sized monsters began collapsing in the streets. They were just children again. Human children.

The entire world was full of humans. Monsters too, yes, but only among an abundance of living, breathing, unchanged humans. There was no Land of Aaa. No Mushroom War had ever happened. It was like something out of his earliest memories. Six hundred years of ruin and strife... had simply never happened. Might not happen.

But the dumb little human who'd dared to play pretend as the Vampire King had seen the world nearly end, in entirely different ways. There was a strange kind of bravery in being so absolutely normal, and still facing the creatures that prowled the night.

Marshall had been small, and weak, when the world had ended. Even if he'd understood what the planes rumbling overhead had meant, he would have been powerless to do anything.

Carefully, he settled himself in the upper rafters of the building. The nearby pane of glass served his purposes perfectly.

"Alright kid, I'm reasonably sure you're going to remember this," he said to a reflection that was gradually fading into view. The magic was dying, and fast. "I can feel you coming back. I remember your fear, and your helplessness. I know you, Xander Harris. And I apologize. But you need this, and I think you'll remember exactly why."

Without hesitation, he brought his wrist up to his mouth. The hiss was more reflex than anything, when his fangs extended and his eyes widened and turned a deep, blood red.

For the first time in ages, he gave into the vampiric biting instinct completely and without hesitation. The fluid he pulled from his own veins tasted terrible, and it was air-temperature like any other vampire's, but blood was blood and the fugue state was undeniable. Marshall Lee's eyes closed -

-and Xander Harris's eyes opened.

It was like waking from a dream. He'd gone out trick-or-treating with the kids, and then they were monsters, and then he'd become...

Oh.

His body was tingling, like it was falling asleep and waking up at the same time. He looked down and, with an audible sucking noise, pulled his wrist away from his mouth. The flesh was pale and raw, chewed into something more like hamburger meat than human skin. The teenager wasn't entirely sure why he wasn't writhing in agony, but a nagging sensation in the back of his head told him about how very unimportant that was, in comparison to the real issues at hand.

He glanced up.

He peered, uncomprehending, at a window pane that clearly reflected the metal beams behind his body. His fingers -the ones not attached to a mangled pile of wrist, anyway- crept upward to the side of his neck. They quested for a pulse, and failed in their search.

Oh.

Even without a heartbeat to drive it, his blood surged with an inhuman steadiness, and blackness came over him.

-The Highschool Library, The Next Morning-

Rupert Giles was weary in ways that the average sleepless night just couldn't accomplish. He'd let out a good deal of his temper last night. More, he was uncomfortable to admit, than he'd thought he still possessed.

Ethan always had brought out the worst in him.

The library was quiet, as it always was when he went about his morning rituals. Lights came on in their usual order, and a small stack of texts waited to be resorted. There were cards to refile and swords to be polished - typical librarian's duties.

"Good morning, G-Man. I really hope you've had your morning coffee." Giles, from his place inside the office, rolled his eyes.

"It is entirely too early for you to be at school, Xander. Let alone awake, I expect. What brings you here?" he prompted from his chair.

"It's about last night. Because of that Ethan guy?"

That caught Giles' attention. The hesitancy in the young man's voice was... uncharacteristic, to say the least. And anything left over from one of Ethan's little 'attempts at fun' was sure to require his immediate attention. He'd hoped they'd all get through things relatively unscathed, too, but it seemed too bloody much for him to hope for.

"Are you alright?" he called out, immediately walking out of the office and into the library proper.

"Sort of. Did you know there are tunnels leading right into the school?" asked Xander, still out of sight. Giles turned, trying to follow the source of the voice. "The person... thing... whatever, that I dressed as last night? It turns out he wanted to be helpful."

"In what way? And, er, could you please come out?" The librarian-cum-Watcher was beginning to feel distinctly worried.

"I already am. Um, look up?" Giles did so, and let out a curse that would have earned a solid black bar on any cable television show.

"Hey." Xander sat on the ceiling, grey-skinned and dressed in plaid, and strummed a red bass guitar that resembled nothing so much as a battle axe. "I think I pulled a Jessie. Do you have a stake?"

-Giles's Office-

The teenager looked miserable. Having assured the librarian that he did not, in fact, want to eat the older man (and had filled up on apples of all things), Giles had felt oddly compelled to invite him... down... into his office. The standard rules of invitations and whatnot that applied to vampires were moot in a public building in any case, and Giles did, in fact, have a thin stake up his sleeve.

"You can drink that?" he asked. Xander looked up from the cup of steaming coffee clenched between his fingers and nodded slowly.

"Yeah. It... it doesn't exactly feel nourishing, or anything, but it's just sort of nice. Like drinking water instead of, well, coffee. It fills you up, but there's nothing in it. Or something." One hand went up and brushed through hair that had become even more messy than the mop that usually topped his head. As if it were defying gravity or moving on an unseen wind.

Even grounded, mused Giles, part of the boy obviously still wasn't.

"And you want me to stake you. You are volunteering to be staked." Xander looked up at this and nodded fervently.

"Yeah. Buffy and Willow could come wandering in and see me anytime. They... they shouldn't see me like this. Not like... me. You could tell them I got eaten last night in the confusion, or something." Xander seemed, then, to shrink in on himself. Like he was expecting to be hit.

"Traditionally speaking, Xander, no vampire is willing to be staked. Ever. Especially to spare the feelings of a pair of, ah, teenage girls. What were you dressed as, last night, exactly? Please feel free not to skimp on the details."

And Xander told him.

He told Giles about Marshall Lee, born in the twenty-first century to a demon mother and human father. About the 'Mushroom War', and the madness of his caretaker, Simone Petrikov. About being bitten and marrying into royalty, before slaying his insane bride and keeping his position by bisecting all challengers with a guitar that could cut through steel.

About the Land of Aaa, and the empty human cities that were overtaken by the fantastic and strange wilderness that the Earth had become.

Giles sat there and listened with an odd, horrified wonder. His drink became cold, and was utterly ignored. Finally, exhaustedly, Xander finished.

"And that's... six-hundred years as a nihilistic prankster. Yup."

"I see. Somewhat." Fidgeting madly, Giles wiped his glasses and frowned at his coffee cup. Where had that come from?

Right, he thought. I brewed that about, good lord, an hour ago? A glance at the clock confirmed it. Hurriedly, he tried to focus on the most pertinent details.

"You say you can, that is to say, you can live off feeding on the color red? I don't believe there's ever been a creature that can do such a thing." Xander shrugged, still looking oddly small and withdrawn.

"From what I remember, vampires in Aaa were more magical beings than actual demons. Except for Marshall Lee, who was half of one before he was even turned. Drinking the color red was like... you know Greek gods?" Giles nodded in some amusement, and Xander flushed, somewhat, and seemed to remember just who he was talking to.

"Right. Well, they had sacrifices made to them from bits of animals, or incense. And that, like, stood for the entirety of their believers' harvests. Right?"

"I'm impressed," replied the older man in honest astonishment. "I hadn't thought you'd have drawn a conclusion like that." Xander smiled, showing a single fang. Giles very carefully did not react. That would not help the boys mental state.

"You've had us do enough research sessions that some of it just stuck, I guess. And Marshall was a quiet bibliophile. He had all sorts of pre-war books stashed in his basement." He shook his head. "But yeah. The red is symbolically close enough to blood that it sustains the magic in them... me."

"Then I don't see what the problem is." At that, Xander glanced up in shock and not a little bit of anger.

"I'm a vampire. You're a Watcher! I'm sure they mentioned, somewhere in the manual, exactly what comes next-"

"You seem to be in full possession of your faculties, including a rather over-developed conscience. You don't seem to need to actually harm anything to survive - less so, even, than your average vegan, I might add. I don't see why this is a problem."

"The sun still burns me. A stake through the heart-" Giles snorted.

"A stake through the heart would do me in, young man. As for the sun," he shrugged. "That's a bit more tricky. Wear thicker clothing and stay away from windows. Become nocturnal, if you don't want to risk it." The older man leaned forward. "This isn't... this isn't like what happened with Jessie, Xander," he said, remembering the young man's recent words and tough life even before he'd settled into a strange new existence as the ally of a Slayer.

The teenager worried at his lip for some time, before finally seeming to stand a little bit straighter.

"You think so? You actually, honestly think that this will turn out alright?"

"I'm not saying it will be easy, young man, but the best way to go about things would be to-" the man stiffened, and glanced over Xander's shoulder. "-Is to run. Really, really fast."

"Demon!"

"Oh shit!"

-Hours Later, In The Library Proper-

Xander's really quite a quick study, thought Giles as he worked through a fresh cup of coffee. Tea just didn't seem like it would cut it, today.

At first, Xander had tried to calmly explain things from a position near the ceiling, in a display of how absolutely useful levitation was in a skillset. Then he'd tried to calmly deflect as many projectiles as possible with his battle guitar, while letting anything else stick in the ceiling. Now, he was just letting the various weapons (stakes, swords, and an entire box of pencils) strike the ceiling and had given up deflecting the things entirely. That was a smart move on the young man's part, given that anything deflected back downward was only recycled in the blond Slayer's attempts to perforate him.

"Buffy, stop."

The girl ignored him, and tried to pierce Xander's heart with an almanac. That, Giles was relieved to note, Xander did catch. Clever boy, not letting the books get damaged.

"Buffy, you will stop trying to destroy my library and kill your friend this very instant!" Finally, it seemed that Giles had gotten through to the young woman. She paused, midway through throwing an encyclopedia, and glared back at him.

"Xander's a demon!"

"Xander still has feelings!" shouted down the young man in question. "I thought we were friends, Buffy. You're being very intolerant of my disability!"

"Disability?! You're a flying vampire thing!" At that, Giles stepped in, again.

"One that helped protect you and the others all last night, even while he was, ah, 'in character'," the man pointed out helpfully.

"I'm still a good guy! My costume was a good guy!" Xander frowned, and considered his last statement. "Okay, he was sort of a dick sometimes, but who isn't?!"

-Author's Note-

So there it is, the first chapter to my first fanfiction. This'll be a three-parter, I believe. Like any good author, I hope that you all tell me exactly where I went wrong or what I managed to do right. I'm aware that I don't exactly have a beta or anything, so input would be very helpful, at this point.

I plan on putting out a chapter of something or other every week, to keep myself in practice. And, of course, to keep you entertained!

Incidentally, "Six String Samurai" is a highly amusing B-movie that I suggest to any and all of my readers. It is exactly as good as Willow describes it!