Xendra

A Buffy the Vampire Slayer/Xena: Warrior Princess crossover fanfic by Diane Castle, author of The Secret Return of Alex Mack, Hermione Granger and the Boy Who Lived and numerous other well-regarded fanfics.

After losing a bet with the Sunnydale High football players, Xander and several other guys have to dress as female characters on a certain fateful Halloween. Xander chooses to go as Xena: Warrior Princess, and finds a chakram prop in Ethan Rayne's costume shop. While Xander's transformation ends when Ethan's spell is broken, it is not permanently gone; it's re-invoked several times -- including when Xander's true form is so grievously injured that changing him back would lead to his very quick death. Xander must then masquerade as Giles' very real niece, The Honorable Xendra Helaine Devaney Giles, while working with the other Scoobies to protect Sunnydale when Buffy runs away after dealing with the demon Acathla.

And that's just the beginning -- Xendra is a massive and engaging exploration of For Want of a Nail against the background of BTVS, with changes at first small but then snowballing until about the only things familiar about the story's version of the series' later seasons are the Big Bads. Different circumstances lead to different developments and different approaches to the Big Bads -- and different consequences for nearly everyone involved. And some familiar faces never appear at all.

It can be found here.

"How am I gonna tell 'em that I'm a magic-wielding lesbian sex-having witch who believes in the supernatural and turned her boyfriend into a girl? And I've eaten bacon!"
 * Anachronism Stew: At least in regards to pop-culture references.  While the story ostensibly starts during the series' canonical time period (1996-2002), the characters routinely invoke, quote or cite works that wouldn't come out until well afterward.
 * Xena's memories, at least to Xander. He notes that she has recollections of events that most definitely did not all take place in a single human lifespan.
 * And I Must Scream: Faith while working for the Mayor, after she discovers that she cannot refuse to kill someone she doesn't want to, no matter how hard she tries.
 * Anything That Moves: Andrea at least gives this impression to the Scoobies for a while after her transformation, but she quickly settles down with a boyfriend.
 * Acquired Poison Immunity: What it turns out that the Cruciamentium is really about -- building up the Slayer's immunity to a number of otherwise deadly toxins of demonic origin.  The immunity is carried by the Slayer Spirit, so even if a particular Slayer dies from the Cruciamentium, the next will have improved resistance to the poison(s).
 * Arson, Murder, and Jaywalking: Willow's panicked babble from the end of chapter 102, at the prospect of her parents both coming home at an inopportune time:

"Andrew handed a small camera system to the biggest monkey demon and sent them on their way. He cackled, "Fly! Fly! FLY!" Lance was looking into a battery-powered monitor hooked up to a receiver. "Isn't it 'fly my pretties, fly'?" "No!" Andrew complained. "That's not what she says! Go watch the damn movie!""
 * The Atoner: Faith, after she's freed from mind control.
 * Attractive Bent Gender: Definitely in play for everyone subject to a gender transformation in this story.
 * The Baroness: Invoked (through G.I. Joe) as a model for Xander/Xendra's early "Goddess Messalina" act.
 * Bavarian Fire Drill: Xander (as "Xendra Giles") when the Council team visits Sunnydale.
 * Also the repeated visits to the morgue to pre-stake vampires during the reign of "Lestat".
 * Beam Me Up, Scotty: Invoked in-universe in part 18:


 * Becoming the Mask: Forced to play the role of Giles' elegant, well-educated niece, Xander finds himself starting to think with her vocabulary and picking up behaviors and tastes alien to his earlier self, and worries that this might be the case.
 * Brainwashed and Crazy: Faith, after evil!Amy tricks her into accepting a "clarity of thought" spell which actually turned up all her bad traits and turned down her good ones.
 * Bratty Half-Pint: How the Scoobies (including Faith) initially see 14-year-old Slayer Kennedy DaSilva.  They call her "Veruca Salt" and other less-flattering names -- at least at first.
 * The Caper: A lot of Scooby plans work out like classic capers, especially the plan to tap into the Sunnydale Army base's comm system so Willow can untraceably crack high-security government computer systems.
 * Chekhov's Gun: There is no dearth of them:  just about anything that initially seems to be a "one-off" event or item eventually returns.
 * Comic Book Time: Invoked at one remove when Xander delves into Xena's memories -- he notes that she seemed to have witnessed or participated in historical events that were centuries (sometimes many centuries) apart.
 * Continuity Cameo: The arrival of a tween-aged Vi (complete with knit hat) on the Summers doorstep as the fourth Slayer at the very end of the story.
 * Crossover: Very loosely.  Xander gains Xena's body, skills, and memories, but after Halloween ends is not her.  And he quickly discovers that while he knows all the ancient languages she knew, not all her memories of the ancient world are accurate either because of different timelines, because modern history is missing information, or both.
 * Frank Gannon came out of retirement to take a job on the Sunnydale police force. (Turns out he's part Brachen demon, and terrified of Buffy.)
 * Dame Not-Appearing-In-This-Fanfic: Dawn, whose creation is one of the things disrupted by Xendra's existence.
 * Dangerously Genre Savvy: "Lestat", which makes him and his followers particularly hard to handle for the Buffy-less Scoobies.
 * Doorstopper: 281 chapters and 960,111 words written over seven years.
 * Double Standard Rape (Female on Male): Averted in the aftermath of.
 * Emergency Transformation: One is effectively performed when Drusilla triggers Xander's third change to make him an ersatz Jenny Calendar with which to torment and torture Giles.  Xander at that point is paralyzed by a spinal injury and not far from death from other wounds; when he is forced back into teen!Xena form all his injuries vanish (but remain, in abeyance, in his male form, complicating plans to change him back).  A couple years later Xander suffers another serious injury and remains female until the necessary ritual can (again) be performed.
 * The End - or Is It?: The story ends with the defeat of Glory, expanding numbers of Slayers, the dawn of a new era of Council support for them... and a conclave of villains planning how to take advantage of the vacuum created by the destruction of the Circle of Thorns, the Senior Partners, Wolfram and Hart, and Glory.
 * Even Chaos Has Standards: Ethan Rayne's patrons -- a variety of gods and goddesses of Chaos -- force him to cooperate with the Scoobies and the DRI to find a way to reverse the effects of his Halloween spell on the children of Sunnydale, because it made it possible for them to become Glory's demonic minions.
 * Fandom-Specific Plot: Starts off as a bog-standard YAHF (Yet Another Halloween Fic).  Arguably it's one throughout, but it's an epic exploration of the idea with some dramatic changes that upset the status quo.
 * Fantasy Gun Control: The Council does not equip Slayers with guns and other modern weapons because of an ancient compact with the demonic forces which keeps the Earth from being overrun outright, and which limits what they may train the Slayer to use against them.  It comes up for renegotiation every few centuries, and the Council only barely got crossbows approved at the last one, several hundred years ago.
 * First Law of Gender Bending: Hits Xander with a vengeance.  First he's transformed into Xena on That Halloween.  Then a spell performed to find the Halloween Xena lookalike briefly turns him into a teenaged Xena.  Then Drusilla uses a stored spell to turn him into teen!Xena again.  While that's undone a few months later, Amy and Willow -- trying to learn how to store a spell in a crystal as Drusilla had used -- find their only success is with yet another instance of the spell to turn him into Xena, which leads to multiple transformations (which Willow or Amy can undo afterward).  It eventually reaches a point where any time the Scoobies need an untraceable female or an extra near-slayer-level fighter, Xander's gender gets toggled.  And after  twice a week.  Eventually the magical principle called the Law of Precedent (the more a particular magic has been performed, the easier it becomes to perform) takes over, and it seems that any sufficiently strong magic in Xander's vicinity will turn him into Xendra.
 * Initially averted with Andrew, who was turned into a teenaged Princess Leia, and wanted desperately to be turned back into a girl because the experience led him to acknowledge that he's transgender. However, he gets permanently transformed as part of undoing Xander's initial stint as Xendra.
 * After getting caught in one of Xander's transformations, Wesley Windham-Price finds that he too is affected by the Law of Precedent and is prone to turning into "Leslie Linda Price". And when he gets involved with Tara Maclay -- a lesbian who is frightened by his male aspect -- he chooses to spend all his time with her as a woman.  At the end of the story he chooses to be a woman full time, and lets his male identity "die".
 * For Want of a Nail: Xander's different costume and Halloween transformation cause changes that quickly snowball, including Jenny Calendar's survival and Oz's death -- and that's just to start with.
 * Foreshadowing: Early concerns about the long-term effects of their Halloween transformation on the children of Sunnydale turn out to be very valid.
 * Fun with Acronyms: The term the Scoobies come up with for the DRI forces before they find out its real name:   Military Operation Futuristic Outsiders, or "MOFOs", for short.  They keep using it for giggle value even after they find out the organization's real name.
 * Gender Bender: Xander/Xendra, Andrew/Andrea, Wesley/"Linda"...
 * Good People Have Good Sex: Willow and Xander (regardless of the gender he's in at the time)... and Faith (only once during the story, though not for lack of trying).
 * Hero of Another Story: In the early part of the story there is an extended sequence where we see the Scoobies working to keep Sunnydale safe while Buffy has run away as in canon, something that was glossed over in the series.
 * Heroic Sacrifice: How Ethan Rayne ultimately breaks the spell that turned children into Glory's minions.
 * Hidden Depths: Faith, who in unguarded moments with Wesley discusses (and quotes) Sherlock Holmes stories, turns out to be a Master of Disguise and is far more complex than she usually lets anyone see.
 * Improbable Weapon User:  Xander keeps up the use of chakrams in combat.  Even though he never gets as good as Xena with them in either form, he does become quite skilled.  They're also advantageous in that most vampires and demons don't know how to handle them, frequently ending up missing fingers when they try to catch the weapons -- and Xander makes his chakrams engraved with tiny crosses for that extra special something on top of their razor-sharp edges.
 * Invisible to Normals/Sunnydale Syndrome: Starts breaking down, at least slightly, in the wake of the Mayor's explosive death in the middle of a park, and the subsequent distribution of tons of snake meat across the landscape.
 * Let's You and Him Fight: The Scoobies use this on the demonic targets that are too big for them to deal with directly, first by manipulating the Circle of Thorns into killing each other out of paranoia, and later by exploiting a magical loophole to drop Glory into the home dimensions of the Wolf, Ram and Hart (the first two of which she kills).
 * Loophole Abuse: A few examples:
 * Willow figures out that the Council's seeming "evil" and dickishness is actually a cover for protecting the Slayer the only ways they can given the demonic compact they are party to but cannot reveal due to its terms, at risk of breaking it. However, the compact had no provisions for third parties deducing and revealing its existence, and when Willow does so... nothing bad happens.
 * Similarly, while the Council is banned by the current version of that compact from equipping the Slayer with modern weapons, it says nothing at all about third parties doing so on their own initiative.
 * The Scoobies figure out that an unanticipated side effect of destroying the L.A. headquarters of Wolfram and Hart could drop Glory into the laps of the Senior Partners -- and so they lure her there and use her to effect the very destruction that then ejects her into the demonic realms.
 * Making Use of the Twin: In-Universe example -- Wesley's female form is apparently close enough in appearance to Faith that with the right make-up they can pass as twins, and in the aftermath of Wesley's second transformation (and the destruction of all his IDs and papers in the process) they exploit this to get back to the United States together.
 * The Mind Is a Plaything of the Body: Xander worries constantly about this even after he's no longer stuck as "Xendra", although after a year or so it finally damps down.
 * Mistress of Disguise: When sharing her identity with female!Wesley to get them both back to the United States, Faith turns out to be very adept at disguising both herself and "Linda", with multiple guises for both.
 * Nail'Em: Xendra uses a high-powered nailgun to slow down and eventually incapacitate Adam long enough for Buffy to deactivate him during the raid on the DRI base.
 * No Endor Holocaust: Averted, at least theoretically, in that based on Xander, Andrew/Andrea and several other people with permanent fallout from That Halloween, the Scoobies are concerned about the possible long-term aftereffects on the children that were also transformed.
 * No Ontological Inertia: Most if not all of the spells cast by the Mayor gradually (or suddenly) collapse on his death.
 * Operation: Blank: When making plans to expand their intelligence on the DRI, the Scoobies spend some time discussing what to call their plans, at one point invoking this trope.  But ultimately this is averted, as they can't agree on any of the words to fill in the "Blank", and simply end up calling whatever they do "The Op", "The Other Op", and "The Other Other Op", as needed.
 * Put on a Bus: Darla, and then Angel, are shipped off to the Vatican to protect them from Wolfram and Hart.  They never return during the course of the story.
 * Reasonable Authority Figure: In addition to Joyce's canon status as this after she learns about the supernatural, Willow's parents also get on board with the team after the truth is revealed to them.
 * Redemption Equals Death: After Ethan Rayne's Heroic Sacrifice is discovered, the Scoobies discuss whether this might be the case -- insofar as what his patrons would consider a redemption, at least.
 * The Reveal: Initially the Council looks just like the Jerkasses and near-villains they are in canon.  But... early on it's unexpectedly shown that many of the "evil" or just plain dickish things they (particularly Quentin Travers) do -- including things like the Cruciamentium -- are forced upon them by or are ways to get around a compact with demonic forces that prevents the Earth from being overrun outright.  The terms of the agreement go so far as to outright forbid telling anyone other than the highest echelons of the Council what's really going on, at risk of breaking the compact and opening Earth up to outright invasion.  For much of the rest of the story the contrast between the Council's real motives and purposes and what the Scoobies think they are makes for an interesting undercurrent to the plot.
 * Running Gag: The early disagreement over the proper collective noun for a group of vampires:  "sparkle", "kiss", or "clot"/"coagulation".
 * Between Willow, Xander and Faith, marshmallow cream.
 * Second Law of Gender Bending: Andrew/Andrea, very much so.
 * Xander/Xendra initially worries about it constantly, but eventually comes to a kind of equilibrium state where he's equally comfortable as either. Of course, Willow being intensely into both of his forms helps a lot.
 * Wesley/Linda is an odd case. Wesley does not come to particularly prefer his female form, but because it makes it possible to be with the woman he loves, he chooses to become "Linda" permanently.
 * Sitting on the Roof: Xander takes to doing this for a few weeks when he's feeling estranged from the Scoobies after Angel returns from Hell.
 * Spoiled Brat: Kennedy DaSilva, the third slayer.  The Scoobies refer to her as "Veruca Salt" and other less-flattering names after they first meet her.  However, Giles and Wesley note that she can turn it on and off at will, and suspect that it may be as much a deceptive act as Buffy's "airhead" and Faith's "sexbomb" personae.
 * To the Pain: "Messalina"'s "torture" of Kevin to get information on "Lestat"'s little band of vampires and followers is almost entirely stagecraft and suggestion.  The only real pain inflicted on him are a couple slaps with a riding crop; he breaks when what he thinks is a dentist's drill is simply brought close to his face.
 * Transformation Trinket: The one-use crystals holding the spell that turns Xander into Xendra.  After a few changes he acknowledges that it's occasionally a necessary and good thing to be able to change if needed, so Xander tends to carry one with him at all times.
 * Transgender: Andrew, who was transformed into Princess Leia on Halloween, very much wants to be back in a girl's body.
 * After a year or two, Xander becomes surprisingly comfortable switching genders as needed for both Scooby business and private fun with Willow.
 * Ultimately, Wesley chooses to become a woman full-time because it's the only way he can be with Tara.
 * Triang Relations: Xander and Willow are open to a Type 8 relationship with Faith, but because of her self-esteem issues and guilt over having raped Xander years earlier they have trouble convincing her of their sincerity.  However, by the end of the story she seems to be coming around.
 * Undead Tax Exemption: Willow eventually acquires the means to create fully-realized histories and identities for "Xendra Giles"/"Xendra Harris" and "Leslie Linda Price", with genuine records in the appropriate government and private databases and all the attendant paperwork, including passports.  Reality Ensues, however, when she realizes she has to falsify years' worth of tax histories and returns for them.
 * Later, she uses the expertise gained then to create a fully-fleshed out legal identity for Angel to help protect him from some of Wolfram and Hart's schemes, and to protect the hotel as well.
 * Unspoken Plan Guarantee: Actively averted with the caper to tap into the Army base satellite dish.  We get all the planning and debugging of the plan, the practice of the physical aspects, and then its execution -- and nothing goes wrong.
 * What Might Have Been: An In-Universe example occurs when Xander is told, in broad, vague terms, what would have happened if the Monks of Dagon had not turned Xendra into the Key; the reader will easily recognize the canonical fifth season of Buffy in the description.
 * Why Don't You Just Shoot Him?: Xander's suggestions (in the guise of Xendra Giles) to the Council members about what to do with Acathla are basically a "good guy" version of this trope.
 * Xenafication: Happens literally to Xander.  Even after he's back to "himself", his female form tends to be stronger, better coordinated, and an all around better fighter -- although nowhere near Xena's level.
 * Yiddish as a Second Language: Willow uses far more Yiddish in this fic than she ever did on the show. Which is not surprising considering how much her parents use.  Amusingly, they tease her about her use of Yiddish.
 * Xander has picked up no small amount himself.