Fate/Accel Zero Order

Fate/Accel Zero Order refers to a an event, part of a collaboration between Fate/Grand Order and Fate/Zero, which occurred during the Human Order Incineration Incident. It involved a small Singularity that manifested in the coordinates of Singularity F in an alternate timeline ten years prior during the Fourth Holy Grail War.

Setting
The Singularity shares a similar background to the Fourth Holy Grail War of the original timeline but with significant differences. Firstly, Kiritsugu Emiya and Irisviel von Einzbern have never met each other nor was he hired by the Einzberns to serve as Artoria Pendragon's Master. Instead she become a proper Master in the war with Artoria as her Servant while he contracted with the World to become a Counter Guardian. So unlike the original Fourth Grail War, the Einzberns prove to be the superior faction. However, the Counter Force sensing that the Einzbern's victory will cause the activation of Angra Mainyu, summons Kiritsugu to stop the war before its completion.

History
Arriving in Fuyuki, Ritsuka, Mash, and El-Melloi II discuss the disaster that will come to the city with the Holy Grail War. El-Melloi II suddenly uses Qi Men Dun Jia to reveal the concealed Hundred-Faced Hassan. He knew she would investigate the rayshift location since she surveys the entire city. After Hundred-Faced is defeated, El-Melloi warns that she will be keep attacking them while surveying the city. Telling Ritsuka and Mash to defeat her other personas immediately on encounter, he and the others move onward.

At the wharf, El-Melloi II reveals All the World's Evils will activate if the require number of summoned Servants are sacrificed. He compares it to a bomb capable of destroying the world, and calls Singularity F the result of the ritual being completed. Mash asks who would make such ritual, to which he answers that was the result of a "man-made diaster". He theorizes that the Grail detected by Chaldea is different from Fuyuki's Greater Grail that had been corrupted by Angra Mainyu. Irisviel von Einzbern and Saber soon arrive like El-Melloi II predicted. Irisviel assumes the group to be participants of the war. Saber notes she no longer detects Lancer's presence, even though he invited them. El-Melloi II reveals Saber's True Name to be Artoria Pendragon and has the group fight her. After Artoria retreats, the group are confronted by Lancer, who was trapped by El-Melloi II's Qi Men Du Jia formation to prevent him from fighting Artoria. He reveals Lancer's True Name is Diarmuid Ua Duibhne, and tells him they aren't Kayneth El-Melloi Archibald's enemies. He tells Kayneth that they came to support him. He gives Reines El-Melloi Archisorte as his guarantor since he can't divulge his name now. Kayneth is convinced to leave the group alone when El-Melloi II sats he'll reveal more and Caster's current location.

After leaving the wharf, El-Melloi II reveals his plan to have at least three Servants withdraw from the battle to prevent the Grail from activating. The group will eliminate the Servants with whom agreement is impossible, such as Caster and Archer. El-Melloi II warns they will likely fight Hundred-Faced when confronting Archer since their Masters are allied. He then says they will have to convince Berserker's Master to join them since direct reasoning with Berserker is impossible. He warns that Berseker may go on an rampage regardless of his Master's order, and warns that his Master is unstable physically and mentally. Deciding to postpone convincing Berserker's Master, the group agree to focus on convincing Artoria, Diarmuid, and Rider with whom they have the best chance.

Later, Mash is prevented from establishing a summoning circle to get supplies from Chaldea due to the leyline being guarded by a spell placed by the Tohsakas. El-Melloi II decides that they must destroy the keystones that power the spell to take control of the leyline for themselves. Romani Archaman worries doing so will anger the Tohsakas, but El-Melloi II reveals they summoned Archer. He then leads the group to each keystone, which knows he the locations of thanks to Rin Tohsaka when she helped him dismantle the Greater Grail in his timeline. At one of the leyline points, the group by ambushed Sea Demons summoned by Caster in an alleyway.

After killing the Sea Demons, El-Melloi II negotiates with Kayneth at his apartment while keeping a telepathic line to the Ritsuka. Kayneth confronts him with the fact that the he was lying that he was acting on Reines' behalf. However, he decides to continue the meeting due to wanting know more of Caster's location from him. El-Melloi II convinces him that he is from the future by stating Kayneth's original intention to summon Iskandar instead of Diarmuid until the catalyst was stolen by Waver Velvet. He also reveals his knowledge that Kayneth's Fiancée, Sola-Ui Nuada-Re Sophia-Ri, to be the one providing Magical Energy to Diarmuid. He then tells him that he will be officially be in Reines' service four years from now. Later, he deceives him that Chaldea's technological achievements were all accomplished by the Archibald family, which were only made possible thanks to his leadeship and founding of Chaldea. After convincing Kayneth to join them, the group head to Caster's hideout in the sewers.

In the sewers, the group eventually confront Gilles de Rais. They defeat Gilles, but Ritsuka wonders where his Master is. El-Melloi II warns his Master is a murderer they must stop. After Kayneth goes to the Church to get an additional Command Spell, the group search for Gilles' Master. Further in, the group find the corpse of Gilles' Master. El-Melloi II suddenly detects a Servant and deploys Qi Men Dun Jia to trap them. The Servant is revealed to be a mysterious Assassin, who El-Melloi II believes to be connected to why 1994 Fuyuki became a Singularity. The group defeat him, but Iskandar and Waver charge in. Their destruction of El-Melloi II's formation allows Aassassin to escape. Waver wonders if El-Melloi II is either Caster or their Master, but El-Melloi II reveals he's a Pseudo-Servant to Waver's confusion. Iskandar tells him that El-Melloi II isn't the owner of the workshop they're currently in. He then tries to pick a fight with the group, but they retreat to his disappointment.

Later, Mash finally establishes the summoning circle, but the group is soon confronted by Artoria. She challenges them to a fair fight, though El-Melloi is unconvinced due to his knowledge of her serving as a decoy while Kiritsugu Emiya ambushes the enemy. As the group continue to fight Artoria, Mash defends Irisviel from Assassin's ambush. He tells Irisviel that he came to kill her because she is the Lesser Grail, believing her death is the fastest way to dismantle the Grail. Artoria then fight him to defend her, which prompt the group to aid her to prevent losing her. After Assassin retreats, Irisviel and Artoria are convinced the group aren't enemies.

The group go to Kayneth's apartment, where Kayneth reveals Berserker's Master has agreed to join them if they help him defeat Archer. El-Melloi II then convinces him that the Holy Grail War is an conspiracy by the Trambelios, the enemy of the Archibalds, to increase their influence in the Mage's Association while he's away. He continues that the Tohsakas confessed in colluding with the Trambelios to create a fake war, and lies that the other participants are decoys. Kayneth then gives Diarmuid to the group after using a Command Spell to stay in Fuyuki and win the war. Outside, Romani warns El-Melloi II that his actions in the Singularity will not change what happen in his timeline as it will disappear once it's resolved. Despite that though, the group continue with his plan and go to meet with Berserker's Master.

Meeting with Berserker's Master, Kariya Matou, El-Melloi discusses with him about defeating Archer. The plan is for Berserker, Lancelot to defeat Archer, Gilgamesh, during his discussion with Artoria and Iskandar at the nearby Einzbern Castle. Interrupting the meeting between the three kings, the group defeat Gilgamesh with Lancelot. The effort however causes Kariya to pass out, which makes Lancelot go out of control and attack Artoria. Not wanting to lose her and having no more use for Lancelot, the group fight him. However, before they can finish him off, Iskandar defends him. He explains he is joining Lancelot to fight El-Melloi II out of sheer dislike and suspicion. He calls El-Melloi II a threat greater than a mere enemy for trying to interrupt the war. He tells Artoria and Diarmuid that they'll be his enemies if they ally with El-Melloi II. Diarmuid prepares to fight him, but El-Melloi II warns they're only continues that El-Melloi is an outside who trying to interrupt the war, which makes him a greater threat than a mere enemy. He then tells Diarmuid and Artoria that they'll be considered if they ally with El-Melloi. Diarmuid prepares to fight Iskandar on Ritsuka's order, but El-Melloi warns they're only allowed to defeat four Servants. Iskandar then leaves, taking Kariya and Lancelot with him, vowing to fight the group once Lanelot recovers.

After Iskandar leaves, Assassin arrives to try to kill Irisviel again. El-Melloi II tells him their goals to end war align, even though their methods differ. He continues Irisviel is Artoria's proxy Master while serving as the Lesser Grail, but she denies being a proxy. El-Melloi II believes Artoria's true Master is Kiritsugu, but Artoria says Irisviel is her Master. Irisviel then shows her Command Spells to convince him she is a Master to his shock. After the group defeat Assassin in an ensuing fight, El-Melloi reveals him to be a Counter Guardian. He reveals the activation of the Greater Grail will destroy the world, and Fuyuki is the cause of the Singularity. Irisviel being both the Lesser Grail and a Master differs from his records, as that type of Master is meant to appear ten years later. Her being a Master with a powerful Servant meant certain victory for the Einzberns, which prompted the Counter Force to intervene. Assassin asks El-Mello II if the Counter Force intervened in the war he knew. El-Melloi II answers the chance was fifty-fifty during the fourth and fifth war. Romani then calls the Singularity a "what if" where Einzberns are the superior faction in the fourth war. Agreeing with him, El-Melloi concludes Irisviel's Magic Circuits are likely what Chaldea read as a Holy Grail, even calling her a psuedo Holy Grail. He then convinces Assassin to help them if it means not killing Irisviel.

Meanwhile, Iskandar captures Sakura Matou from the Matou Residence while burning the worms in the basement. He agrees to give her to Kariya in exchange for his right hand and Command Spells, which he concedes to. After serving his hand, Iskandar has Mithrenes take Kariya and Sakura to a hospital outside of Fuyuki before leaving. After he leaves, Zouken Matou finds his manor to be burning and a contract-less Lancelot. He then contracts with him to get revenge.

At the Greater Grail's cavern, El-Melloi II reveals his plan to safely dismantle the Grail. The original plan was to destroy directly, but that was abandoned since they failed to recruit Iskandar. Suddenly, El-Melloi II and Asssasin detect Hundred-Faced. After defeating her, the group proceed to the Grail's chamber, where they fight Iskandar. However, after Iskandar is defeated, El-Melloi II is severely wounded by Lancelot on Zouken's orders. Zouken wishes to witness the Grail's activation despite Irisviel calling it a failure. Wanting to defend the Grail, he commands Berserker to attack the group. However, with Berserker's defeat, the Grail is ready to be activated, and Zouken extracts magical energy from Irisviel to hasten its awakening. Suddenly someone resembling Irisviel emerges from the Grail designated Black Iri. After devouring Zouken, she goes to devour the rest, saying five Heroic Spirits aren't enough to fill her. Diarmuid and Artoria hold her off while the others escape.

Outside of the chamber, El-Melloi II uses his remaining strength to make Waver Zhuge Liang's new temporary vessel. The group then return to the chamber to help Diarmuid and Artoria fight Black Iri. After she is destroyed, Diarmuid and Artoria disappear since the Grail was destroyed. El-Melloi II announces the Grail detected by Chaldea is gone. He asks Irisviel what she will do now since she can't become the Grail any more. Mash asks her to return with them to Chaldea since their original goal was to retrieve the Grail. She accepts the offer, and El-Melloi helps Waver leave the cavern. Assassin and Irisviel realize they have a strange yet pleasant connection. He is satisfied he was able to protect something without sacrifice. After they thank each other, Irisviel rayshifts to Chaldea with the others.

Later, Assassin tells Angra Mainyu to come out of the Grail. After emerging, Angra Mainyu asks Assasin why he didn't warned the group of him since they only destroyed the vessel and not him, the contents. Assassin answers he didn't want to rely on others from another world to destroy him, whose curse solely belongs to his own world. Angra Mainyu criticizes him for continuing to be a Hero of Justice even after death. Assassin has no regrets for his chosen path though, saying they'll both curse and save the world respectively. Before fighting him, Angra MAainyu asks him for his name, to which he answers Kiritsugu Emiya.

Participants
Servants