Disclaimer: Sherlock Holmes and properties are owned by Sir Arthur Conan Doyle. Sherlock and properties are owned by BBC One, Mark Gatiss and Steven Moffat. Harry Potter and properties are owned by JK Rowling. I don't make money from writing this fanfiction. The story contains spoiler for both Sherlock and Harry Potter. Also there are quotes from novel, TV series, wikipedia and other sources.

Everlasting Moon

Chapter I

Oh, no! No, no,no! Sherlock Holmes thought frantically as he tried to save his best friend, Dr. John Watson. Sherlock didn't return to this. He didn't spend three months destroying Moriarty's crime empire so he could keep his friend safe only to witness John died in front of him. Technically, Mycroft's men did the most work since Mummy was furious with the whole Moriarty thing. And Mycroft, always eager to please his beloved, quickly wiped out the threat to his younger brother.

Sherlock and John just returned from the crime scene. Who would have thought that the assassin had crept into 221B Baker Street through a window in a mission to kill them? The assassin failed and was killed in the process. But before that, the assassin managed to hurt John with poisonous dagger. And now, John was lying on the sofa with pale blue face, ragged breaths and cold skin.

"John!" he shook his best friend's body frantically. "You can't die! You can't die yet!"

John's lips were moving. I'm sorry, Sherlock. He said.

No! It wasn't John fault. What should Sherlock do? He couldn't let his best friend die. Something, anything. He had to save John, no matter what happened. Making his decision, Sherlock ran to his bedroom to find an antidote for the poison. He had kept it in secret place. Place that Mrs. Hudson and John couldn't find. There it was! He grabbed the vial and ran back to the living room where he force fed John the antidote.

The effect was instant. Colour returned to John's face. His breathing also went back to normal. Wrinkles faded from his face. And from experience, Sherlock knew that every scar on John's body also healed.

The doctor opened his eyes. "Sherlock?" he asked hesitantly.

John was alive. All that mattered to Sherlock was his best friend was still alive.

John slowly resumed sitting position. He looked at his hand and paused. His hand looked younger. And not only his hand. He stared at his reflection on the nearest surface. He also looked younger and healthier. His shoulder also didn't hurt anymore. What had Sherlock done?

"What did you feed me, Sherlock?" he asked. His voice was calm, like always when he was under pressure.

"Yes, Sherlock, do tell." Came the annoying voice of Mycroft Holmes.

Sherlock groaned. His elder brother was really fast this time. "Go away, Mycroft!" he barked out.

Mycroft ignored him. "Do you have any idea of what you have just done?" The elder Holmes asked coldly. His expression tight with anger.

John had never seen Mycroft looked like this.

"I have to save John," Sherlock replied in equal cold tone.

"You compromise Mummy's safety!" Mycroft exploded.

"I would never endanger Mummy!" Sherlock replied, enraged by Mycroft's accusation. How dare Mycroft was to suggest such thing. Sherlock loved Mummy dearly. Mummy was the one who raised him ever since he was three years old. Mummy loved him like a son. Sherlock wasn't like Mycroft, who lust after Mummy ever since he hit puberty.

Mycroft took a deep breath. "This isn't a place to discuss this," he said at last.

"No, it isn't." Sherlock replied curtly.

Mycroft glanced briefly at the dead body of the assassin who tried to kill him and Sherlock. "I have to take care of this too." He drawled.

"Well, you have to be useful somehow." Sherlock said snidely.

Mycroft looked at him coldly. "My car is waiting outside," he said.

"Lead the way then," Sherlock replied.

"What is going on in here?" John asked. He was confused by the Mycroft and Sherlock's behaviour. They were hiding something from him.

"Later, John." Sherlock said.

The journey to the safe house was tense. Both Sherlock and Mycroft were staring at each other coldly. John was confused as hell. What was going on in here? And more importantly, what did Sherlock feed him? John felt ultimate cold. He couldn't move this body. He honestly thought that he was going to die. But after the mysterious drink Sherlock gave him, then he suddenly was fine. The drink tasted heavenly. It wasn't like anything John had drank before. And the effect... he felt warm, alive, energized. He had never felt anything like that before.

The car stopped and John switched his thought to his present surroundings. They got out from the car and entered the magnificent mansion. Their destination was Mycroft's study. Or at least, John assumed that it was Mycroft's study.

"I'm going to call Mummy," Sherlock stated suddenly. "Mummy would understand."

Mycroft gave Sherlock withering glare.

Sherlock ignored him. He then left the study, probably to call his mother.

"I'm afraid that I'm needed to explain Sherlock's reckless act to Mummy." Mycroft said after a few moments of silence. "Now, if you'll excuse me."

John was left alone.

Fifteen minutes later, Mycroft and Sherlock returned. They looked better, at least for Sherlock part. His flatmate looked relaxed and happy. Mycroft still looked annoyed.

"I apologize for the waiting, John." Mycroft said. He was always the polite one.

"Would any of you explain what is going on in here?" John asked. He was tired being left in the dark.

Mycroft gave him a measuring glance. "Well, at least he is trustworthy," he drawled out.

"Of course John is trustworthy!" Sherlock said.

"I'm sitting right here," John muttered in annoyance.

"Elixir of Life," Sherlock said suddenly.

"What?"

"The Elixir of Life saved your life. Otherwise you're going to die."

"Elixir of Life?" John asked, frowning. "Like in those movies?" The elixir of life, also known as elixir of immortality and sometimes equated with the philosopher's stone, was a mythical potion that, when drunk from a certain cup at a certain time, supposedly granted the drinker eternal life and/or eternal youth.

"Yes." Sherlock confirmed.

"Is this a joke?" he asked.

Sherlock and Mycroft stared at him.

"Okay," he said.

"I must say that his reaction is better than what I hope for," Mycroft drawled.

"It's the shock talking, Mycroft." Sherlock commented.

John glared at both of them. "After spending two years as your flatmate, I think I'm quite immune to everything now." He had been kidnapped by criminal mastermind and strapped with Semtex. He exhaled hallucination gas at government facility. His best friend faked his death for three months. John didn't think he could get shocked now.

"It's a logical conclusion," Mycroft said.

Sherlock gave his brother a dirty look.

"Does the elixir mean that I'm immortal now?" John asked because he had to know. The drinker always became immortal in the movies.

"No," Sherlock replied. "You will never die as long as you drink it."

"Surely you understand that this is a secret on the highest level," Mycroft said.

John nodded. "But, what is your mother connection with all of this?" he asked in genuine confusion. Sherlock and Mycroft kept talking about their mother.

"I got the elixir from Mummy," Sherlock explained.

"Is she a scientist or something?" he asked.

Sherlock and Mycroft exchanged amused look.

"He is a wizard," Sherlock supplied.

John blinked. Was Sherlock serious? This was too much for John. Was this a dream? John secretly pinched his arm. It hurt. He pinched his arm again. It hurt. Okay, so this wasn't a dream then.

There was a knock on the door. It opened to reveal a handsome young man with amber eyes and jet black hair.

"Hello Sherlock, Mycroft and Dr. Watson," the newcomer greeted them.

John merely nodded.

"I'm Teddy Lupin. It's nice to meet you, John. I've heard so much about you." Teddy said.

"Teddy is my god brother," Sherlock explained. "We grew up together."

Teddy nodded at that. That was new information for John. Mummy, god brother, what was next?

"Teddy," Mycroft interrupted.

"Sorry, Mycroft," Teddy said. "Well, are we ready then?" he asked.

"Yes," Mycroft drawled.

"Good," Teddy said. He took out a ballpoint from his pocket and held it.

Sherlock and Mycroft touched the ballpoint.

"Well, come on, John." Sherlock said. "We're going to see Mummy." He sounded happy at that.

Confused, John touched the ballpoint. What were they doing anyway?

"To the eternal city." Teddy said and a second later, they were whisked away.

Author's Note:

This is my seventh Sherlock Harry Potter crossover fic. In this fic, Mycroft and Sherlock were raised by Harry but they weren't his biological sons. Being ten year old genius with mature mind when Harry began to take care of them, Mycroft never ever once considered Harry as his parent, unlike Sherlock who consider Harry as his parent.