Nothing much to say, except this Shirou is not Shirou Emiya, but Shirou E. Custos, a 'Shirou Emiya' that born from the negative aspects of Shirou Emiyas from every other universe. He still wants to be a hero though, but in his own unique way. He does not possess any abilities his counterparts have but an 'altered' version of Time Alter and a couple other ones. That's all I can say for now to avoid further spoilers. And please don't say something like 'this makes zero sense', because if I do follow Nasuverse's rules or real life logic, this would not be a 'fan fiction'

~Turn 0~

Unlike the others, he knew of his unusual origin and role in this forsaken world. Born as the personification of negative emotions from all of 'his' in other dimensions and forever been cursed to become a 'villain' while retaining all memories from previous lifetimes, that was his fate. Ironically, a so-called 'King of Ruin' like him would always be named Shirou Emiya Custos with Custos meaning 'guardian'. He was about to accept his fate and wholeheartedly determined to become the King of Ruin until he met them.

"-Custos. Sir Custos!"

"Wait what?!"

Shirou was snapped out of his thoughts as he looked around fanatically like an idiot, making his fellow knights laugh. He was attending the weekly meeting between the Knights of Round and the King to discuss about problems in Camelot or just simply for trivial matters. Today, King Arthur-errr Artoria was proposing a grand party to celebrate the victory against the major force of the Saxons which forced them to sit and agree to cease fire with no strings attached. Shirou frowned; yet another foreign 'point' in the history that he unwillingly caused. He did know of the Arthurian Legend and the aftermath but his helping sir Gareth when he, err no, she had been surrounded by the Saxons had earned him a position in the Camelot army. Rumors around that he singlehandedly slaughtered that whole Saxon group, which was almost true since he had technically 'killed' them by absorbing their essences but leaving the bodies intact. Shirou was okay with rumors as long as they didn't affect his personal life.

"You seem to be bothered by something lately. Does it hurt to share with your wife?" Gareth held his hands with concern in her eyes. Yes, he was married to Gareth Beaumains, who was now Gareth E. Custos, despite his disagreement due to his special circumstances and untold origin. Well, Gareth was a stubborn girl and she always reminded him of a twin-tailed girl he had met during his visions of the other counterparts. It's not like he hated her or anything, he actually felt pity for Gareth to even fall for the King of Ruin. Speaking of marriage, he was shocked when finding out that in medieval era, people could have multiple wives. Guess what?

"While we agreed to share him, I disagree with you spending more time with Shirou than I do sir Gareth." Artoria spoke up with rather irritated tone toward Gareth who shot a devilish smirk.

"No! You and Gareth keep snatching Shirou from me. That's not fair at all!" Mordred cried cutely at the seat opposite to Gareth's. It had been a common thing in Camelot that Sir Custos captivated the hearts of the only female members of Round Table and constantly being the reason of their bickering. Shirou had changed this timeline significantly, like consulting with Lancelot about his affair with Guinevere while advising him to tell Artoria the truth or helping mending the relationship between the King and Mordred. While he was satisfied with the results, Shirou in turn created an unknown 'point' in the timeline by accidentally making Gareth, Artoria and Mordred fall in love with him.

'This cannot continue any further. I must fix this or Gaia will bring her wrath upon me, or worse, on those three.'

Shirou had thought this through: he would sever his tie to this world by planting the seed of doubt and hatred toward him into the people in Camelot, thus lowering the chance of Gaia unleashing her wrath upon this beautiful kingdom. He even wrote an alternative way by arranging his death so that even if the people still loved and respected him despite his carrying plan A, he wouldn't exist to alter the timeline any longer. He offered to the criminals in the jail to spread news of him, Sir Custos, committing grievous crimes such as raping, murdering, etc not them in exchange of their freedom. No need to say, they accepted quite fast and used every way they could think of to spread the news as wide as possible.

Approximately three months later, the people began losing their faith in Shirou and the Knights were divided into two factions: one was demanding his execution for tainting the title Knight of Round and the other was defending him, saying that those were malicious rumors. Much to his irritation, Gareth and Mordred were on the latter and trying their best to track down whoever spread those rumors. Artoria, in other hand, was surprisingly silent. Regardless.

'So plan A was a failure after all.'

Shirou then went to meet Galahad, who was fiercely against the rumors due to his being Shirou's apprentice and thus understanding his mentor better than most.

"Galahad, I assume that you don't believe in those rubbish news, no?"

"Of course! A man of honor like Master will not and never commit such things! I swear to you as one of the Knights and your student that I shall find and bring back the head of the culprit to you!"

Shirou smiled sadly at the declaration. He put his hands on Galahad's shoulders and pulled him closer within range of whispering. Shirou then told his student of everything including his origin, knowing that Galahad would not betray him by speaking them to the others.

"No way… Master, you can't possibly do that!"

"I must. Do you still remember the story about the rise and fall of a kingdom I told you back then? It is the true story about Camelot without my presence and interference. Lancelot is one of the major reasons leading to the fallen state of Camelot. Mordred rebels against her Highness due to her not recognizing her being her 'son'. You are Lancelot and another woman's son. There are more factors that lead to Artoria's death and the fall of Camelot. But thanks to that, the world can become better and people remember you, the Rounds and the King as force of justice while Lancelot and Mordred are the force of evil."

Shirou's sad smile began to be visible.

"But due to my presence here, there is almost no evil in this world aside the Saxons, who we defeated that day. As King of Ruin, my duty is to become the Evil so that the Balance between Good and Evil can last forever. Do you understand?"

Galahad didn't say anything and just nodded.

"Good. Now, my last requests to you…"

…..

"Absolute no! Even if you are the King of Ruin or whatever, I cannot do that to the person who taught me how to be a good Knight!"

"Now now, it's not like I will 'die'. I am cursed to be immortal so after the execution, I will lay down for a time and then I will meet you again. No need to worry. Oh, if you have the urge to tell my wives of this plan, please exclude the whole 'I am King of Ruin and I am immortal' thingy. They cannot handle it."

"…Yes, Master."

Shirou smiled as the blade pierced through his heart and Galahad kicked him into the pit below. His student briefly nodded to him, which he returned the gesture, before turning back and declaring that 'the disgrace to Knight of Round is dead' blablabla. Shirou, having mended the hole on his body, used Time Phaser to teleport him to the edge of the cliff. At there, he set a small camp for the time being while establishing a link to Gaia to report back to her that he had 'settled' the matters. The Mother of Earth was satisfied and called back the order to attack Camelot from her 'Killing Machines'.

*From this are the events from the 6th Singularity in Fate/Grand Order but the Knights and Lion King's reason for the whole thing is to gather enough souls to revive Shirou. Gareth actually hung herself in grief which explained her disappearance during the Singularity*

"- I am sorry, Artoria, Mordred, Gareth." Shirou stood before the makeshift graves he made for his 'wives'. "It was my selfishness that caused you so many troubles. While I would be happy to stay here with you all, I am forced to follow my mission."

A rift shredded the space and time around him, announcing that it was time for his duty. Shirou strapped his sword, the Phaser, a parting gift from his student and Lancelot, who also taught him the art of Eternal Arms Mastership prior his demise at Agravain's hand. He touched the headstones for the last time before entering the rift and disappearing.

Crappy but reflect Shirou's immature way of handling the situation. From now will be Shirou's adventure in DxD world as the King of Ruin. I will actually elaborate why he is called King of Ruin and how he fights without his Trace On ability. Note that his Eternal Arms Mastership is two ranks lower than Lancelot's, meaning he has to hold the object for a period of time like an hour or more to efficiently adapt to it.

And please less flames and more contribute reviews! I can't blame you if you find this chapter a bit crappy though.