I don't own these series of course. Doom is owned by ID Software and Fate is owned by Kinoko Nasu at least I think he still owns it.
The skies were a mixture of black and red as the people of Fuyuki city ran in fear as a monster tore through the city. The ritual had been complete, Avenger had been born. Angra Mainyu was finally able to punish the world for what they had did to him. He could make them feel the pain he did when they forced their evils upon him for all those years. He casually pushed over a skyscraper as he waltzed through the once peaceful city. Even in the modern age humans still felt the need to build large structures to prove their so called superiority to everything else that existed. He could feel the fear, desperation and squish of those below him as he destroyed them. It was a euphoric feeling really, was this how his tormentors felt when they used him to release their sins?
His reminiscing was cut short however as he felt a pain upon the side of his head. He looked over in the direction the pain came from to find a lone human standing upon the buildings and aiming what appeared to be a gun at him. This human stood at 6 feet and wore a green suit of futuristic armor that covered its whole body. He was amused at first that a mere mortal would stand up to him but when he truly gazed upon this human, he felt it. The hatred, oh the hatred. He could tell that it was as much perhaps even more hatred than he himself had. This was a being of pure hatred, not just a human but a spirit, a heroic spirit and judging by the power he sensed, it was a grand spirit. It made sense of course, for his summoning would result in the calling of a grand servant but he wasn't expecting one such as he. This heroic spirit was not any of the normal sort but an avenger, like he himself was. The audacity really, to send an avenger to fight an avenger? If he could, he would write a complaint to the counter force about their sick twisted humor.
The armored spirit began to fire more bullets at the beast, they were small but were still leaving a mark on the newly born abomination. Angra slammed his massive arm forward into the building as the armored man flew violently through the air and landed upon his arm. He took out what appeared to be a chainsaw and rev'd it. He plunged it into beast's arm and began to run upward, dragging the demented weapon as unholy blood began to rain from the sky. He screamed out in pain and with his other arm, simply flicked the annoying servant away as he went crashing through several buildings.
With his now bloody arm, he threw more blood to the ground as monsters began to form out of it. They were horrific creatures born from his hatred and they thirsted for blood. They all began to charge towards the servant in the hundreds, no, the thousands. After reaching his location they began to dogpile him and tear him apart. Angra watched in delight but his moment of laughter was cut short as a green explosion erupted from the pile of demons as a massive blood geyser erupted from their location. Walking out of the bloody pool was the servant, a peculiarly large gun in hand as he stared menacingly at the beast. The gun disappeared as another appeared in his hand, a smaller gun. A double barrel sawn of shotgun. In his left hand, a hilt appeared. The hilt opened up as a red energy like blade with a runic language on it appeared.
The man began to run towards the beast with an ungodly amount of speed. His blade tore through monsters like nothing, as his shotgun blasted them apart. Its blazing barrels of brimstone spew his vitriol and loathing upon them and casted them into the dirt. He stepped onto the head of one and crushed it as he flew into the air, fire blasted from his feet and he flew even higher. A winged creature came to intercept him but his diabolical musket fired a chain from underneath it, its claw of pig iron gouging the flesh of the beast as he used it to swing closer to his target.
Angra was tired of this game, he punched at the flying the man only for him to swing his swords in response and cut straight through his fist. He was taken aback by this, so much so that the man flew forward and plunged his sword into the head of the beast.
What was he? He was an avenger but instead of hating humanity he was fighting for it. Like some avenging angel, here to save humanity from its sins. He could feel his hatred through his attacks, his fury far surpassing his own. Was the beast perhaps feeling fear in this exact moment? Was this fear due to the losing battle or the fact that the creature he was fighting was all he was or could have been?
The man pulled out a large Warhammer, made from the same energy as the sword. He jumped into the air, flipped forward two times and slammed his hammer into the sword as it flew through the beast's head, shattering it in two. The beast fell backwards and crashed into the ground violently.
His now wide apart eyes looked downward to find the armored man walking up his torso. He stopped at his chin and stared down at the beast. Angra stared into the man's eyes. They were filled with nothing but rage and hatred but that rage was tempered, controlled to be used for its one purpose. To rip and tear, until it is done. The man pointed his very large gun at the beast as it began to charge up for what felt like an eternity, the beast's eyes began to shake and dilate with fear until finally it fired out a green ball of energy that collided with the beast and let out a large explosion that blew the dark clouds away.
As the light died down, all those who were still around could see a large shadow imprinted into the ground where the beast once laid. The slayer of the beast stood there, looking up to the sky as the rays of sunlight began to rain down once again. His mission was complete, the beast was dead and he saved mankind from the brink of extinction, again. Although there was one last thing he needed to do before he left. He went to site of the ritual to find those responsible, only to find them already dead. The man who started the ritual and the girl used as the catalyst laid motionless on the ground. The culprit was a large armored man wearing a cloak with a skull for a face. They both nodded to each other in silence as they finally faded away from existence, returning to the throne to be called upon once more to save humanity.
End.
Authors notes: I've always wanted to write a Fate fanfic but I was afraid of getting something wrong about the lore because I know how passionate Fate fans can get about the lore.