This has some things from Fate/Grand Order: First Order like Cu Chulainn being Caster and Medusa being Lancer. The Rider is Alexander the Great from Fate/Zero.
This IS a fourth route for Fate/stay night.
Even with some minor changes, just go with it. And during the 3rd night, Rin was on the rooftop because she sensed the faint presence of a Servant, and was going there to check it out after school had been let out.
The Holy Grail War. Every 60 years, seven mages are chosen to be Masters. Each summons a Heroic Spirit, known as a Servant, to fight for them. The Servants fall into one of the seven classes.
Saber.
Archer.
Lancer.
Rider.
Caster.
Assassin.
Berserker.
Once all seven of the Servants in each of these seven classes are summoned, the seven Masters fight to the death in the city of Fuyuki in Japan. The last Servant-Master pair left standing is awarded the Holy Grail, a cup able to grant any wish of the Master and Servant. Each Servant has a chance of winning. Even if they lack another's raw power, they can still win a battle against an enemy Servant by outsmarting them. It has been 10 years since the end of the last War. The fifth one is soon to begin...
The Church was a dark place. Only a few candles and the moonlight lit the area. A priest with short brown hair, brown eyes, and wearing a Church uniform faced the podium at the front of the room. In his left hand, he held an open book. This man was known as the Overseer for the War. Unknown to others, he was a Master in the War as well.
Behind him and to his right, a man sat in one of the front pews. His left arm was slung over the back of the pew. The man had blonde hair and a black jacket over a white shirt. He looked human in those cases, but he also had red eyes with serpentine pupils. Even the air around him gave off the impression that he wasn't human.
"Where is the dog known as Caster? He has been absent from the Church for a few days," the man said. The priest closed the book and stood up straight. He replied without turning.
"I sent him out to gather information on the other competitors. The other day, a girl named Rin Tohsaka informed me that she had summoned an Archer Servant. I need Caster to check out her Servant's fighting abilities once the school she attends lets out. I instructed him to let out only enough mana for her to detect it and draw her in. If she's smart, she'll wait until dusk falls to check it out. Like with the other Servants, Caster will fight it to ascertain it's fighting abilities."
The first chuckled. "By showing himself to the others, his chances of winning the War will be lessened."
"It does not matter how his chances will be once the final Servant is summoned. His job is simply to give me information on the others. That way, your chances of winning the War will be greater, Gilgamesh."
"You slayed his original Master. What was her name? Bazett? You promised him another chance to win. And now you're going back on your word and throwing him away?"
"Despite how I see him, he is still a Servant in this War. Depending on how he fights in it will determine whether or not he will have a chance of winning. Bazett simply was not able to make full use of his abilities. She would have been killed along with him right away. It seemed like such a waste to allow someone like him to die right away. That is why I stole Bazett's Command Seals and became Caster's Master instead. However, his use to me will come to an end once he reports to me on the fighting abilities of the Saber."
"What will you do with him then, Kotomine?" Gilgamesh asked. The priest grinned.
"I will break my pact with him so I am free to make a new one with you. Will that be acceptable?"
"It will be, but the problem will be Caster's action towards you after the pact is broken. He still harbors a grudge towards you for killing his original Master. Once he has been freed, he will most likely kill you."
"That is why I will ensure my safety to you."
Gilgamesh sighed. "Very well. I will keep him from harming you. But what will you do for me in return?"
"I shall give you Caster's fate after I break my pact with him."
Gilgamesh raised an eyebrow. "What do you mean by that?"
"To prevent himself from fading away, he will rush to make a new pact with another Master. If he grows attached to that person, ripping that joy away from him will be amusing. I shall allow you to witness his despair at losing his Master a second time at the hands of the same man."
Gilgamesh laughed. "You never cease to keep me amused, Kotomine! It shall bring me great joy to watch that dog suffer."
Kotomine turned around so he was facing the other man. The priest smirked. "I had hoped that would be satisfactory to your tastes."
Gilgamesh was still chuckling. "Very well. If you can promise me that, I shall allow you to give me orders during this War."
The night was still, save for a car that occasionally went down a road. No one who happened to be out driving paid any attention to the figure leaping over rooftops. The figure was a man. However, his clothing suggested he wasn't from this era. He wore a grey-black shirt over his torso. Matching glove-like garments covered both of his arms. They cut off at the knuckles and only held onto his hands by the fabric over his palms. Armor ran down the length of the top of his arms and ended at the top of his hands. On the index finger on each hand was a silver ring. Over the shirt looked to be a light blue hooded poncho that wrapped around his back and over his arms. A white strap held both end together over the top of his chest. The garment itself cover over half of the man's torso and to the elbows of each arm. The ends looked tattered. The hood was lined with white fur over the edges. The garment was over his long, blue hair with only the ends of it visible past the end of the garment. Over his waist was a white belt holding the white cloth under it up. Beneath the cloth was what looked to be the bottom of a cloak. Like the other garment, it was light blue. Over his shins were metal guards and on his feet were sandals.
However, he didn't seem human just like Gilgamesh. Like that man, he had red eyes with serpentine pupils. Long, silver earrings hung from each ear. Even the air around him gave off the impression that he wasn't a person, but rather some kind of beast. The scent he gave off suggested that as well. He wasn't human, but a Servant. He was Cu Chulainn, a Servant that fell into the Caster class.
Caster wasn't happy. He had to fight two Servants in one night. During the one against Archer, there had been a witness, a red-haired boy who ran inside the school building to avoid Caster as the Servant hunted the boy down. He finally caught up to the kid on the third floor. The lad had ran fast, but was no match for the speed of a Servant. He fired a spell at the boy, boring a hole through the kid's chest. That should've killed him. But later that night, Caster had to kill the same person. The lad had fought back with rolled-up paper, but it must've been strengthened. The lad blocked each swing of Caster's staff like he was holding a sword. The kid had jumped into glass to the outside trying to get away from the Servant, but Caster's kick sent him flying near the shed. The kid struggled to his feet and ran inside the shed. He turned around just past the doorway and unrolled the paper like a shield, trying to block a downward swing from Caster's staff, which was covered in flames. The paper couldn't withstand it and burst, floating through the air as white scraps.
He was about to finish the lad off once and for all when a blinding white light filled the room. He had just enough time to block something swung at him with his staff and was thrown out of the shed. It turns out that the kid he was trying to kill was a Master. He had blocked something completely invisible, which he assumed was a sword, by the kid's newly-summoned Servant, Saber. Not only had she been able to react fast enough to avoid an arm of Wicker Man, it gave away his true name to her. Right then, Kirei called for him to return to the Church. He left Saber with a threat as he retreated, bounding over rooftops towards the bridge.
He growled as he headed for the Church. Even though he had been ordered by Kirei to only analyze the other six Servants' strengths, it seemed that Saber's Master was unable to give her enough mana for her to use her full strength, thus it should have been easy to kill her. It would mean one less person they had to fight in this War.
It was strange, though. Kirei was revealing Caster to all of the other Servants, so now they all know what to expect from him, eliminating the element of surprise. Why would Kirei do this? Did he have some plan that the Servant hasn't been made aware of?
He finally reached his destination. The Church loomed before him, feeling dark and foreboding. All was silent, save for the noise one cricket was giving off. Inside gave off the feeling of the place being a dungeon instead of a church. No lights were on. Only the moonlight filtering through the windows cast long shadows from the pillars and pews as it spilled over the floor. His own footsteps echoed off the walls, alerting the false priest at the front of the room of his presence.
The man's voice sent chills up Caster's spine. "Ah, Caster. I'm glad you have finally showed up. Now you can explain to me why you used your Noble Phantasm."
"The seventh Servant, a Saber, appeared," Caster replied. "She caught me by surprise. I was left with no other choice but to use it. We were in a stalemate otherwise."
Kirei sighed. "You know you should be prepared for anything." He looked at Caster. "How are her fighting abilities?"
"She fights close-quarters with an invisible object, most likely a sword."
Out of sight of Caster, Gilgamesh perked his head up. An invisible sword? It's hard to believe she would be summoned two Wars in a row, but that must be her.
"Anything else?" Kirei asked.
"Not much, but she is incredibly strong. I also got the feeling her Master, the boy ya ordered me to kill, was unable to give her enough mana fer her to use her full power. Despite that, she managed to keep me on my toes while I was fightin' her."
"Very well." Then Kirei grinned. "You're usefulness to me is gone now."
It took Caster a moment to process what Kirei said. "What do ya mean?!"
It happened in an instant. Several golden portals appeared around the Servant. Chains flew out of each of them, holding the man tight. They coiled all around him, holding his limbs, neck, and torso. He struggled to free himself, but to no avail. The chains wouldn't even budge. "What the hell is this?! The fuck are ya plottin'?!"
Kirei lifted the sleeve over his right arm. Caster's eyes widened. It was covered in Command Seals. He lifted it up, facing Caster. "By my Commend Seals, our pact is terminated."
One of the Seals flashed red before fading. Caster's body went limp as the mana that had been flowing into him was cut.
Kirei lowered the arm, letting his sleeve cover the Command Seals again. "Caster, I suggest you flee from here right away and find a new Master before you vanish. Also, don't even think about attacking me. I do have another Servant, and I will order him to kill you if you attack."
The chains vanished and Caster fell onto the floor on all fours, breathing heavily. "Ya...were workin'...with...another Servant...this whole time..?!"
Kirei folded his hands behind his back. "It will be amusing to see how much you struggle throughout the rest of this War. The next time we meet, I expect you to be ready to fight me to the death."
Caster chuckled. "Don't get...too full of yerself...Kotomine...Even like this...I could...kill ya easy..."
"Unless you want to die, you shouldn't do that just yet. Now get out of here. I sense Tohsaka and another approaching the Church."
"...Fine." Blue mana started to cover his body. "But never stop watchin' yer back, 'cuz I'll be comin' fer ya!"