I've read a few different Fate stay night fanfics where Shirou develops a different skill set or origins. It's always fun to see it happen, And I decided to put my own spin on it. What if he inherited a symbiote, and would develop powers similar to Venom.
In terms of plot, I took inspiration from 'Fate Hammer Time' and 'A Mage's Guide to Being a Vigilante.' good stories both worth a read, even if the former is one of the longest fate fics I can think of. I won't be using any Canon Symbiotes, like Venom, Carnage, or the like; mainly OC Symbiotes. The story takes place in Nasuverse, just with the addition of Klyntar Symbiotes in the universe.
Also, don't be surprised if other protagonists from Fate verse start popping up later on. But not the ones you would expect, especially for a Fate Stay Night fanfic. Most FSN characters will be, mostly in character, but others… we'll see.
Magus Killer, meet Alien Symbiote
The fire burning around him was from hell. The building was destroyed by the living corruption that burned everything it touched. He estimated that hundreds of people died from it, if not more. Kiritsugu Emiya assumed he was the only survivor. The scorched bodies he passed by didn't imply otherwise.
The Corrupted Grail caused this. Its contents poured out onto the area, burning any humans upon contact. Saber's Noble Phantasm failed to destroy it completely, causing the fire surrounding him.
He walked, gazing at the fires. He walked, gazing at the corpses of men who died that night. He stood in the Everything he worked for, the sacrifices, the hardships, all in vain. His
The Magus killer shed tears. 'What was the point of it all? To end here?' He wanted to die, but started walking instead. He walked into the dark night. He walked as the fire burned around him. He walked as his heart ached, both from the despair he felt and the curse the Evil God had dealt him.
Durring his wandering, he noticed movement from the corner of his eye. Turning around, he saw a young, red headed boy, walking as well, the boy was weaker then he was, and fell. Kiritsugu acted on impulse, he went towards the boy. He couldn't save anyone before; maybe he can succeed now.
Gazing at the child, he never noticed the metallic goop that slithered out of the debris nearby. He was too busy holding the child to notice it sense him, dive towards the skin of his exposed leg, and phase into his skin.
Kiritsugu saw that the boy was injured, so he acted fast. He removed Avolon from himself and placed it into the boy. when he fealt a chill go through him, he assumed it was from removing the fae artifact. The child spasmed, and coffed, intaking air, and opened his eyes.
Kiri looked into the eyes of the one person he managed to save. He couldn't save his wife, or assistant or daughter, but he could save this child from the hell he had a hand in unleashing.
The Magus Killer picked up the child, and started walking out of the burning wreckage. For some reason, his bones stopped aching and he found a newfound strength in his being. Even the fires started to die down as he walked out. The darkness faded away in the coming dawn. A new day meant he could live. He saved one life, maybe he could save his daughter as well. Only time would tell.
_The Next Day_
The burning area from the Grail started cooling down a few days later, but its mark was still felt, and would be for years to come. Kiritsugu could sense the remaining curses lingering in the area. Other people just sensed raw, like the area itself would harm them if they stayed. People started calling it the great fire of Fuyuki, appropriate, since it burned a portion of the city so badly that. The lingering curse of the Grail was still there, unnerving any who entered the area.
Kiritsugu approached the boy he saved in the hospital. According to the doctors, he lost his memory due to the trauma of the fire; no family, no friends, nothing. Any family he had most likely died in the fire. He approached the boy, introduced himself as a magus and offered to adopt him. The boy, Shirou, said yes.
Getting a house was simple; he already set it up after speaking with Raiga Fujimura, the local head of the Yakuza in Fuyuki. With any luck, Shirou will have a home, a father, and a big sister if Raiga's granddaughter takes a liking to him.
Now, all that was left was to deal with the… creature that had latched onto him during his walk out of the fire. He didn't notice it until the next day; a mistake he planned to rectify.
After setting up Shirou to pack their new home, with Taiga looking after him, he headed into the city. He found an empty warehouse, one he recognized from earlier in the grail war. He set up a few small bounded fields to keep people away.
Kiritsugu looked down at his hands. "I know you're there. Show yourself, or I'll make you." He honestly wasn't sure what would happen next. He's dealt with spirits and demons a few times but not enough to know all there was about them.
What he didn't expect was a mass of shining metallic ooze to start seeping out of his shoulder, forming a serpent-like head that looked back at him. it had pure crimson eyes and a maw of sharp teeth, with which it spoke. "Hello Kiritsugu Emiya. I-"
The Magus killer did what came most natural to him; he drew his Thompson Contender and shot the metallic face. The face was blasted away, but reformed seconds later. He fired a second, third, and eventuall sixth shot, emptying the barrel of his gun. It reformed after each shot. He reloaded his gun, this time with one of his last Origin rounds.
"Wait!" the metallic face yelled out, its voice sounding eerie with a slight echo. "You don't want to do that! Right now I'm bonded to you, if you use that origin round, it could kill us both!"
"...You think that would stop me?" Kiri loaded the origin round and aimed at the face again.
"..." the metallic face said nothing at that, reconsidering how to approach him. Then it knew what would appease the infamous assassin. "I can help you reunite with your daughter."
Kiritsugu kept his aim true, but didn't fire. Yet. "Explain. What are you? How did you infiltrate my body? How and why would you help me?"
The face smiled, showing its metallic teeth. "I am a Klyntar, an alien species from another world. We're sent out into the galaxy to find a suitable host, one we bond with. Many of our numbers were corrupted, cursed by an ancient evil, becoming feral and spreading across the galaxies like a living plague. Thankfully, I am not one of them, and retain my reason, my mind, and my honor. It is our sacred task to-"
Kiri pressed his gun to the creature's head. "Skip to the part that I care about."
The face flinched. "We can empower our host's bodies: enhanced strength, speed and stamina, regeneration, heightened senses, and other abilities we develop depending on our hosts and what we consume. Allow me to stay, and I can help you get your daughter from the Einzberns."
The Magus killer thought about it and realized that no one would know his plans for the Einzberns. Unless… "You've been reading my mind since you bonded with me. That's how you know about my past." Kiritsugu surmised.
The Klyntar nodded. "Yes. we can view the memories of our hosts. I ended up bonding with a few humans before you. I learned a few human tongues from their minds."
Kiritsugu mulled that over for a minute. It was unheard of in the normal world, and unthinkable in the Moonlit world. Even so, it is possible the creature is telling the truth, about his origins at least. 'Strictly speaking, Dead Apostle ancestors and their kin qualified as aliens, so his kind could exist in galaxies beyond this one. Even still…' "How did you end up attached to me?"
"I remember falling to the earth and being brought to a facility. They captured me and a few other Klyntar and were doing research on my kind. Mostly involved having his bond with animals, then human hosts. When I refused to leave a host, they killed it and dragged me out to a new one. But then…" he tilted his head to the window outside.
The Magus killer caught on. "The Fire from the grail destroyed the facility, giving you a chance to slip out. You slithered around for a host, and found me."
"Correct." the Klyntar smiled, lesser men would run in fear at that smile, but Kiritsugu thought nothing of it. "I know all your abilities and skills, your resources and plans, all of your experiences. I was never bonded to a magus, but I feel I can improve your body, as well as your magecraft. We can even develop new… methods to improve ourselves."
"Can you remove the curse that's going to kill me?" The Grail didn't take the magus killer's rejection kindly, leaving him with a lethal curse that is slowly killing him. He's not sure how long he has.
The Klyntar frowned. "Sadly, no. I tried eating the curse, but it forced me to belch it out or suffer it as well. I can strengthen you, make you stronger and faster then you were at your prime, but I can't stop the curse from killing you. If you used that gold relic you had before, I could prolong it indefinitely, however… your son's body may not take kindly to that."
Kiritsugu shook his head. "Avalon stays with Shirou. He'll need it, more than me."
The head nodded, before it started retracting into Kiritsugu's body. Kiri was about to question why, until he heard the alien's voice in his head. 'Easier to talk this way; no one can hear us.' The assassin nodded, houlstered his gun and walked out of the abandoned warehouse.
Kiritsugu had already decided to keep the creature. "You mentioned others. Were you the only one of your kind in the facility?"
"No. There were two others in the facility, but both of them died in… the fire." His tone was somber.
"But you didn't." Kiritsugu countered.
"My kind are susceptible to fire, but I seemed to have developed a sort of…" he mulled over how to explain it. "Affinity for it? I seem to be able to absorb heat and flame to grow and survive. It's how I survived the fire before I bonded to you."
Kiritsugu walked down the main street. 'Any other traits you have?' he switched to mental discorse so no one noticed him talking to himself.
The Klyntar seemed to smirk in his head. "I think I can produce and modify metals. I could produce temporary metal tools and trinkets while bonded to past hosts. Given your weapons of choice, it seems ideal."
'Hmmm…" He headed. "We'll need to test our limits, see what you can and can't create, and implement new methods. If you can produce heat and metal, then weapons and firearms might be something you can produce. bullets might be something you can make. Combined with my experience in making bombs and traps, it will prove useful.'
The two creatures - an assassin magus killer and a warrior alien symbiote - continued their discussion. From there, a partnership would be born that would set the stage for Kiritsugu had failed many people in his life. He wasn't sure if he'd ever see his daughter, but he'd make sure his son would outlive him.