I can guess the hell I'll get from this, but it's been on my mind since Jack's debut in the Fate/Apocrypha series so I'm writing it. Before anyone asks, yes this will be a JackxOc story and so will it's probable sequel.

Darkness and Light, Into the Chaos.

Trifas, three days after the beginning of Jack's slaughters.

A young boy followed an old man down the street. "Master, why are we here again?" the boy asked.

"Simple, young Asiko, we are here to investigate the deaths of the residents and our fellow Magus. We suspect a Servant in the Holy Grail War is the cause, so we must be on our guard." the old man replied.

Asiko nodded, looking around and trying to sense any mana in the area. The sun was setting and the blonde thirteen-year-old was sure the Servant would appear soon.

As darkness fell, a fog began rolling in. "Asiko, wait." the old man ordered, "This is no ordinary fog."

Asiko waited, and heard a giggle. "You catch on fast. We're impressed." came a girlish voice. There was a blur of purple and black as the old man fell dead. "And what about you? What should we do with you?"

Asiko was shaking. "Please, I don't want to die!" he cried.

"Oh, you're not very old, are you? Hardly worth killing." the voice remarked, sounding almost disappointed. Asiko saw a figure approaching. When the figure came into view, the young Magus was shocked. Standing in front of him was a girl not much older than him with light purple hair, golden eyes, and pale skin with a scar across her right cheek. She was wearing a black vest and stockings, and holding two knives as long as her forearms.

"Beautiful." Asiko whispered despite his terror. The girl blushed slightly at the comment, turning to hide it. "Who are you?"

The girl turned back and replied, "Our name is Jack the Ripper."

Asiko recovered from that shock pretty fast and asked, "Well, what now?"

"What do you mean?" Jack asked, tilting her head slightly in confusion.

"Well, what are you going to do now that my master's dead?" Asiko clarified.

"Probably grab his heart and go home." Jack admitted with a shrug. "Mom isn't a Magus, so we have to feed on others to maintain our form."

"What if I helped?" Asiko asked, earning another look of confusion from the Servant. "I could supply you with mana, I could send word that my master killed you and died in the process to get the Magus council off of you. What do you say?" What am I saying? Asiko asked himself. He was offering to help a serial killer.

"Would you? Really?" Jack asked sweetly. "Would you help us?"

"Why not, just as long as you promise to not kill anyone else who isn't a Magus or Servant." Asiko replied.

Jack nodded enthusiastically. "Okay!" she replied, turning around as the fog vanished. "Come on! Mom will want to meet you!" And with that, still questioning his own sanity, Asiko followed Jack into the night.