It's the spookiest time of the year, and all at Chaldea are eager to celebrate! All but one of course, Rostam being much too old and distinguished for that childish dress-up-and-eat-sweets business. The bridge team all wonder aloud if he's going to be visited by the ghosts of Halloween Past, Present and Future next. Speaking of which, Marche wonders where that shota Servant version of Rostam from last Christmas went before being abruptly cut off by Alfons and Shinji to mention that tonight's the costume contest. At this point everyone flees to do final touch ups, leaving Rostam grumpy and alone as usual. Later in his room Karna makes one last effort to convince him to join in, which Rostam resists - albeit he doesn't say 'no' to a teasing suggestion that he model his costume in private instead. Karna leaves to compete in the contest and Rostam is grumpy and alone a little more, musing that it already feels like he's in dress-up playing pretend 24/7.
At the contest one random mundane staffer named Karina suspiciously has a character sprite of her very own. She's excitedly putting the finishing touches on her witch costume, only to be dismayed that that bitch Gina from Analysis came as a sexy witch with her giant titties flying out everywhere. Lamenting that Halloween is about drinking and being an asshole in public while dressed like a hooker rather than the pure experience of cavities and stomachaches because being an adult has to make everything suck, a despondent Karina slips out of the party without entering the contest and runs away to her room. She almost knocks Shinji over on the way out, who came as a horse's ass - part of a pantomime horse with Alfons that is.
The scene intercuts between the climax of the competition, Rostam internally debating if it would still be gauche to at least show up to the party if he has no costume, and a mysterious presence contacting Karina in her room. The silhouetted figure promises her that together, they can bring back the true spirit of Halloween. Karina accepts, and Chaldea shakes ominously as the contract is sealed.
Receiving panicked messages from several Servants at once, Rostam races to the common room to find Karina on stage in her completed witch outfit, albeit no mere 'outfit' now. Rostam senses a power from her akin to his, a Demi-Servant - no, a pseudo-Servant, an existence that cannot be materialised by the Grail but takes a human host to become a facsimile of one. Karina proudly proclaims that she's come to restore the spirit of Halloween by force, and create an unforgettable night that lasts forever. She releases a wave of energy which curses all present in Chaldea to fuse with their costumes and teleports away, laughing maniacally. Being the stodgy no-fun man that he is, Rostam finds himself the only one unaffected. Pandemonium ensues as all Servants and staff present find themselves mentally corrupted by their costumes and acting out wildly. Rostam retrieves Alfons and Shinji (now a centaur and satyr respectively) from a tide of sexy nurses that are in no way qualified to be wielding that many syringes and beats a hasty retreat.
Tasking stock of things, Rostam realises that they picked up an extra him on the way out. It's Reynard, having cosplayes him for the contest, using his 'Chaldea casual' sprite with slitted green eyes and fox ears. He rushes Rostam in a panic asking him if he always feels like this and wails that he's going to lose his mind. Karna has many choice words to say in response, but having been transformed into a solid gold statue by his body paint they sadly fall on deaf ears. To make matters even worse, Trismegistus contacts the group to tell them in no uncertain terms that this whole affair is stupid and he's not going to help. Rostam, lacking any better ideas that don't require infinity Command Seals, suggests tracking Karina down and forcing her to reverse the curse. Karina astrally projects at the mention of her (she thinks it was her name but Rostam confesses he has no idea who she is, much to her consternation) and taunts him that only one with the true spirit of Halloween can breach her barrier.
A trip to her witchy abode soon proves this to be true. Marche has been shanghaied into being her gate guardian, functioning as a Shielder with the same sprite and abilities as in classic GO. Only a one-on-one duel can break the barrier, but Rostam quickly finds himself outmatched by her Defence Up stacking and the battle ends once five turns have passed. She tells him to return once he has unlocked the true spirit of Halloween, and no sooner. At this point Karina astrally projects again to offer her services in the event shop, because if you're trying to get into the spirit of the season you might as well pay her in sweets.
As the group muse over what the true spirit could mean, Karna suggests returning in costume. Rostam blushes furiously and demands they find another way, while his cursed fellows angrily shout at him to just fucking do it already. With much trepidation and coaxing, Rostam finally relents and gets changed in his room... only to scream as the curse wave retroactively affects him. The others burst in (using the Karna statue as a battering ram) to discover Rostam fused with a sexy rakshasa outfit, evoking the Dangerous Beast CE design of Halloween 2016 in classic GO. Rostam braces for mockery, only for Shinji to point out that as a satyr he is currently feeling the overwhelming urge to fuck everything and everybody so he's in no position to point and laugh. Reynardstam, meanwhile, wails that he's too stodgy and humourless to even laugh at Rostam properly. Karna says something that makes Rostam blush and throw his jacket on over the top of everything. This unlocks the first temporary copy of the welfare Servant, Halloween Rostam of the Alter Ego class.
Rostam of course wants to get this over with as soon as possible, but Alfons mentions that in his weakened state from the class-switch he's in no shape at all to battle the level 80 Marche. Karina appears to add to this (after a pregnant pause as Rostam's new form distracts her) that the key to wearing a costume is being the costume, and that Rostam cannot possibly possess the true spirit of Halloween if he's ashamed to be seen dressed like a skank. The team decides that they have to defeat the many other cursed Servants and staff around Chaldea to strengthen Halloween Rostam for the final assault. Karina gets unusually into it for being the supposed villain, eagerly offering rewards in exchange for milestones such as beating up vast swathes of generic sexy nurses, sexy nuns (a challenge battle with Kiara), sexy police officers, sexy witches, sexy vampires...
The quest is fraught with many perils. Orochi is collected without incident, having been cosplaying a human at the time, but Spartacus is given a wide berth as his BDSM gladiator gear has become a full hooded gimpsuit and he makes extremely disturbing noises any time someone so much as breathes near him. Siegfried appears wearing a blue longcoat and declaring that he's a demon hunter. When asked what exactly he's dressed as, a misunderstanding ensues in which Rostam thinks he failed extremely badly at cosplaying Empress Nero, which enrages Siegfried so much he manifests Sanson Alter's wraith and attacks the party as an Avenger. Halloween Rostam must take part in this and all other event story missions and Shielder Rostam is locked out. Immediately afterwards Corday appears, having been dressed as a vampire to fish for bonus irony points. Now sharing a sprite with Carmilla for maximal irony points, she attacks the party and is defeated. Once both are defeated they come to their senses somewhat and become story supports, still bearing their modified sprites and, in Siegfried's case, class.
Next is Sanson, having dressed as a clown for the contest. The party is so unnerved by his behaviour that they attack him immediately - he is considered Beast I class and is named 'IT!'. However, after the battle he reveals that he was never affected by the mental pollution of his costume, and was genuinely trying to be fun and friendly in misunderstanding of the holiday. The group extract a promise from him to never do that again, and allow him to come along. He too gets to keep his changed class as a story support. Remus is the next opponent, having been bleached bone white by a prank of Reynard's and is now a massively muscled and furious Berserker Poodle. He only slightly calms down after the battle once it's pointed out that Rostam has been even more humiliated (much to his consternation), and Reynardstam beats a hasty retreat rather than risk existing in the same postcode as Remus.
These battles all drop copious amounts of All Class XP with which to level Halloween Rostam, and Karina gladly offers him (rubber) severed heads as ascension materials for each level milestone, yet Rostam still feels no different. Shuten appears to bar the way, having assumed the form of an adult and androgynous version of his GO design. Carmilla!Corday grows amorous at the sight of him/her, which incenses Siegfried enough to attack. That Fitela soon appears dressed up as Sigmund is only gravy.
After the battle, an increasingly incensed Halloween Rostam sinks his fangs into the meat of Shuten's shoulder after one final tussle, leaving him confused and shocked. Shuten is into it if course, and admits that he wasn't hit by the curse at all - he just shapeshifted and played along because he wanted to fight and Halloween is the silly-buggers season. Rostam points out that he doesn't normally need an excuse to be a shithead, to which Shuten counters that yeah he's the exception that proves the rule. He mentions that oni and rakshasa are not so dissimilar, being shapeshifting beasts of very human wants and urges, and Rostam should use that if he wants to get into character. Rostam replies that that is staggeringly unhelpful. Shuten is entirely unbothered.
The final penultimate obstacle turns out to be Vlad Lily (having come as Himselves) and a miserable Andersen in a duck costume. The latter bails on the former immediately, 'defeating' himself so the farce can be over quicker. Vlad Lily has four health gauges, cycling between all three personae before finally being defeated in his true form. He speaks about the nature of all three of his alternate forms as scarecrows of some kind, and how everyone's entitled to one good scare on Halloween. Rostam muses he may have a better grasp on the spirit of the season now, and departs to battle Marche a second time. In this solo battle redux he gains a large NP Gain Up buff, which helps him spam his Ignore Defence NP and overcome Marche's buff stacking.
Her guardian defeated, Karina astrally projects again and asks Rostam if he truly knows the spirit of the season. Rostam replies that he doesn't, because it's a stupid holiday with even less of a coherent image than Christmas, and exists mostly to just do whatever. Enraged that Rostam fucked her around, Karina charges out of her territory and just attacks the party. Rostam responds something along the lines of HA HA TRICKED YOU FUCKER and an even angrier Karina calls on Medea - now in black and with green skin - for backup. Battle ensues in which Medea supports a three-health-gauge Karina, who shifts Ascension stage each time until -dun dun dun!- her third gauge reveals her to be Circe, and ends the battle.
Circe admits that the spirit of the season stuff was all bullshit and she just wanted to hitch a ride in someone compatible so she could troll everyone to get the last word over that whole Okeanos business. Karina begins to resurface, insisting that Halloween is important, and during this internal conflict the bloodlust seizes Rostam again. He advances on CirceKarina and threatens to eat them alive as punishment for the curse they cast over Chaldea. Circe holds firm but Karina seizes control in a moment of panic and breaks the curse, freeing everyone. Rostam reveals that this, too, was another 'trick or treat'. Karina exclaims excitedly that this was her favourite Halloween ever, growing somber as she adds that with so much on the line and every new day uncertain, she wanted live the night up like it was her last. Alfons assigns Karina cafeteria and janitor duty for a month to make up for all the lost man-hours and general disarray, albeit adding that if Circe is agreeable she can also work off the debt in the field. Circe replies that she never intended to stay even as long as this, but riding along is so fun she might just make it a permanent thing.
In the epilogue, Alfons debriefs Rostam about the data that Hermes (begrudgingly) collected. Everyone seems to have reverted none the worse for wear except Rostam - as a Demi Servant he seems to have modified himself to the extent that he can assume his Halloween Rakshasa form once again at will. While he thinks of this, Rostam asks if Alfons ever got around to scanning Marche. Alfons replies that of course he didn't, she was just costume-cursed like everyone else. Rostam drops the matter with a few random mutterings. Meanwhile in a darkened corridor, Karina's silhouette promises to keep 'her' secret leaving it ambiguous as to whether Circe or her host is speaking.
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