Chapter 1

#1) Night of the Blue Moon

Most magus know that vampires originated from Type-Moon. Although there is one exception recorded only as a fairytale that was kept within the clocktower and certain magus families. There was a vampire that was born from not Type-Moon but a Blue Moon. Although it is confusing and something that no one will ever understand, some speculated it was a phase shift. Meaning the Blue Moon and the Moon shared the same space just in different layers and occasionally the Blue moon would overtake the layer and shine its light. The vampires cursed and hunted down the newly born vampire. All because the blue moon was a symbol of disaster and misfortune. 

The Vampire of the Blue Moon, Vanitas, was chased away and weathered the winter storm by themselves. The vampires unable to see Vanitas any longer ceased their chase only for the light blue eyes of Vanitas to pierce through the snow. In their hatred and fury they created a cursed grimoire, the Book of Vanitas. A blue leather cover and jet black pages, a clockwork grimoire linked to a silver chain. "Oh hear me foolish vampires, upon opening this book, a fate worse than death awaits you. Whoever holds this book will hold the same eyes as me and inherit my power." 

Inside a gloomy barren forest held a True Ancestor. What was he doing? Running away in fear. The sound of a parade had begun to echo throughout the forest. Fog started creeping up through the ground and blurring the surroundings. Normally True Ancestors were one of the most powerful species on Earth, yet they like any other beings had a weakness. Their true name, the very foundation of their being. As much as he disliked it, he needed help and that was why he had swallowed his pride to call for the current successor of the Vampire of the Blue Moon, who was currently posing as a doctor for whatever strange reason. 

Until then, he had to survive and run. "What a nice place~ Can you give this place to me?" A raspy sinister voice appeared from the fog as the True Ancestor did whatever he could to stall the entity. A black foggy body, facial features were non existent except for glowing red eyes and a smile. 

"NON!" 

The True Ancestor breathed a sigh of relief as a clanking of gears echoed throughout the world,a clear sign of the arrival of Vanitas. A black gloved hand ripped through the entity with ease as blue sparks drove the creature away. The hand that had ripped it apart had altered the world formula in order to hurt it. A screech was let out as it disappeared. 

"Now I'll be needing that fee you promised me. I'm moving to a different place. If you ever need me then contact me through that drunkard or Merem. It seems that thing still isn't satisfied." 

A book was pulled out by the teen as the True Ancestor trembled, the descriptions from the fairytales all matched, it was truly the Book of Vanitas. 

"I am Vanitas! The successor to the Vampire of the Blue Moon, yet I am but a normal human! No cower in fear and humble your pride as I save you." 

Sparks of blue formula alterations crackled around the book as the clockwork grimoire opened itself to the world. In an instant a burst of blue light expanded throughout the forest. Flowers that glowed blue started to bloom and the fog disappeared. "It'd be best if you lived here from now on, I've made it so that this forest is no longer accessible to that thing." 

The True Ancestor nodded before handing over the promised payment. "Never let anyone steal your true name. You were lucky "Bellus" that your true name was not yet taken." 

The True Ancestor's eyes widened upon hearing his true name. "It's quite a strange name but I do suppose you have to have a few quirks to stand out." 

With business done, Vanitas waltzed out before jumping onto the rooftops of the nearest town. Staring at all the humans going about their day not realising the true extent of the dangers lurking every corner. Especially maguses, he detested them just as much as vampires. The only reason he was like this in the first place was due to that retched magus. 

Vanitas laid on the rooftop of some random house as the pupils in his eyes shifted into diamond shapes. Mystic Eyes of the Blue Moon, eyes that allowed him to alter the world formula at will. Although he didn't usually use them much unless a severe inverse operation called for it. The world formula gave him peace as he stared up at the night skies before falling asleep on the roof. 

-------------------

//Fuyuki//

Vanitas fiddled a bit with the hourglass earring on his left ear. A late momento from the Luna, the one who had saved him from the damned magus. The one who taught him everything he knew. And that led him to Fuyuki, info from the True Ancestor's networks had told him that someone was trying to bring the Vampire of the Blue Moon back. He wouldn't allow it. 

To stop that from happening, he would have to destroy the grail in a roundabout way in order not to attract attention. While the grail war was some small scale ritual in the east, it still holds some weight within the clock tower. 

Vanitas headed towards the cave in the grail as the clockwork echoed in the background. Sparks of formula flew as the Book of Vanitas worked its way to alter the formula of the grail itself. To summon something that should have never been summoned. 

An extra class of great destruction. 

An avenger, a class that was only known to the info keepers and the original creators of the grail war. 

"Begin Calculations, Adjusting World Formula, Altering foundations" 

The sparks of formula flashed as the grail was tampered with beyond fixing. Yet in all that, a bright light flashed as dust rose up and a searing pain manifested on his right hand. Command spells gleamed in the shape of a three point star. From beyond the smoke, a woman stood tall, well as tall as she could be. 

Short grey hair, long black cape and armor, a sword black as night and golden eyes. 

"Servant Avenger, Jeanne D'arc. Are you my master?"

Vanitas's eyes widened as he smirked, "You're pretty fierce for a short stack~" A wave of cursed fire was sent towards his way only for blue sparks to fly as they vanished. "Hmph! At least you have some semblance of strength as my master." Sparks continued to fly around Vanitas as Jeanne froze up. 

Behind Vanitas was something only she as a spirit could sense. It was horrifying, a dark haze surrounding Vanitas. Gleaming blue eyes and cracked mouth, claws that seemed to reach out to her face, to warp her very existence. Her breathing was hitched, she was going to be die moments after being summoned. 

Only for a wave of the gloved hand to dissipate the illusion. "Looks like you can see it. The curse of the blue moon." Vanitas looked towards the city before smirking back. "We do things my way, we use my method and if you don't like it, then I'll dispose of you as well. Capiche? I have a feeling this war is going to get very dangerous. Especially with that present in this city." 

Vanitas walked out of the cavern as Jeanne followed him. "I don't like being called Master so just call me Vanitas instead."

The moon shone bright without any stain of colors, however, the color would change soon. A very dangerous prediction. Vanitas knocked on a house. that seemed quite innocent. "Kiritsugu Emiya, I require a place to stay." 

The former Magus Killer stared warily as the footsteps behind him bolted out. "Vanitas! Why didn't you tell us you were visiting earlier?" The long white hair whipped Kiritsugu in the face as he was pushed out of the way. 

Vanitas held out his hand before Jeanne materialised as Kiritsugu and Irisviel realised the implications. "Come in and talk." 

Vanitas and Jeanne sat down as Kiritsugu started firing question after question in order to get a sense of the problem. Vanitas stared at the wall before smirking "Looks like you two have a handful~" Irisviel giggled before calling the two children hiding behind the wall out. 

"Shirou, Illya, come out and meet Vanitas. He's the one who helped us all those years before." 

A red hair boy and a white hair girl that looked almost identical to Illya emerged from their hiding spot. Kiritsugu simply looked like he had enough of life. 

"So, let me get this straight. Some insane magus is trying to bring back the Vampire of the Blue Moon using the holy grail or even the summoning ritual. While also at the same time, it's going to be a red moon soon meaning it will be here looking for more names." 

Irisviel covered her mouth as the implication that it would be here was more than enough to address the situation. It was already a shit fest with someone trying to bring back the Vampire of the Blue Moon that has had their true name warped but also Naenia, She Who Harbors Death, was also going to make an appearance. Kiritsugu could only lament Fuyuki being a cursed land that brought nothing but misery. 

"I'll help out in this war, I'm assuming you've already locked the grail?" 

Vanitas nodded, Jeanne had been the trigger he used to interfere with the Grail in order to prevent any future wars from ever being held. The Grail had lost it's ability to absorb mana from the leylines permanently. 

"The guest room is to the right of the hallway."