* Disclaimer: Fate/Zero is the property of Konoki Nasu, Type-Moon, Aniplex, and Infinite Budget… I mean UFOTABLE. (I mean do these even really matter, do big anime companies and mangaka have divisions dedicated to scouring fanfiction sites and taking down stories without disclaimers? It doesn't really matter because no sane person reads these things, eh fuck it, might as well just leave it in). *
Not Beta'd
The Holy Grail War, a magical event that puts seven masters of seven heroic spirits in a battle royal for the prize of a single wish. These wars are bloody and fierce and there has never been a victor, and no single war matches these themes better than the fourth Holy Grail War, the fourth war was an event chalk full of blood, despair, death, and betrayal, or at least it would have been if one master didn't make some drastically different choices.
The opening of the war is exactly as you would remember, the war opens with a duel between the Saber, Artoria Pendragon, and the Lancer, Diarmuid Ua Duibhne, that is so rudely interrupted by the oaf of a Rider, Alexander The Great, who then proceeds to call out all of the other servants and masters for being cowards hiding in the shadows, which draws out the Archer, Gilgamesh, and the Berserker, Lancelot, and the master Kayneth whatever the fuck his name is, does anyone really remember this fop of a character? Though there is one small difference, there is no assassin on top of the crane.
You see in the original canon war Kiritsugu Emiya the master of Saber spotted the assumed to be dead Assassin watching the bout from atop a crane, which ruined the whole ruse of Assassin being killed off by archer, but in this war Kirei had Assassin use presence concealment so the Magus Killer was completely unaware until the knife slipped into his heart, said knife was also thrown through Maiya's skull, just in case.
In the confusion that was felt throughout the battle field as Saber suddenly exploded into golden pixie dust as she died the dominoes began to fall, the Assassin that Kirei had hidden in the Tohsaka household thanks to Assassin's noble phantasm went into the study and ended the life of Tohsaka Tokiomi, and the brief shock Archer felt at his "masters" death was finished with a knife decapitating him, had to watch out for that pesky battle continuation, and there is the end of the most arrogant, egotistical prick pair of master and servant in the war.
Meanwhile in some dank, dark sewer the Caster of this war, Gilles de Rais, rushed away from his twisted master to enact revenge on the ones who ended the life of his precious "Maiden", the poor delusional little psychopath, Ryunosuke was confused at the brief exit, but shrugged it off and returned to his art, a lovely piece using the entrails of a poor young widow he had recently kidnapped, he didn't even get the chance react as his life was ended by a knife to the back.
One Matou Kariya was watching these events from the end of the docks and while at first, he was quite confused, he was overjoyed when he watched the death of Tokiomi's servant, so caught up in his joy he didn't think of the consequences of these events until the knife slipped between his ribs. Also, Kayneth was stabbed in the back at some point as well, but not even the Assassin that did the deed remembered him after they killed the fop of a professor.
Now all that was left was Rider and his young master, so Kirei had a small number of his Assassin's rush the pair, but they were quickly caught in Rider's reality marble, the Assassin's proceeded to summon the rest of their ilk from their noble phantasm, but were quickly steam rolled by the might of the Macedonian army.
And that was how the fourth Holy Grail War ended in a single night, with Rider and Waver as the victors and since their victory was legit Zelretch decided to pop on over and pluck Angra Mainyu out of the grail for some experimentation or some other fucked up shit that goes on in the clock tower, so Waver and Rider got their wishes and went off on some other adventure now that rider had a real body.
The women of the masters, on the other hand, had drastically different fates from each other, Sola Ui, was finally free of that shitty marriage to that piece of cardboard Kayneth and while she lost that hunky piece of man meat that was Lancer, she still counted it as a plus one and went on with her life, Irisviel, on the other hand, had keeled over from a seizure after the death of so many servants dying in succession as her body broke apart.
And that is how some different choices from just one of the masters changed the fourth Holy Grail War completely.
And that's my little one shot that I made after being confused by how poorly Assassin was used during Fate/Zero and doing some thinking on it. I originally intended for Kirei to win the whole thing, but I couldn't honestly think of a way to kill off Rider and Waver due to Rider's strength and the fact that they're always together in Fate/Zero, so I just decided to let them win. And it wouldn't be a crack fic with out an appearance from the resident troll.