Disclaimer: This story uses some ideas from the Harry Potter and the Deathly Hallows Part 2 movie and some concepts from the anime Full Metal Alchemist, which are, of course, not mine.
The Resurrection
It was a foolish thing to do but Severus Snape's mind was not in its usual effective and methodical state that night. Right after feeling the sharp, excruciating pain on his arm where the Dark Mark was, Snape Apparated to the Godric's Hollow, the place where Voldemort would murder the young boy who was foreseen to be the cause of the Dark Lord's downfall. Only select members of the Death Eaters' Inner Circle knew this, and Snape was "privileged" to receive this piece of information because it was he who told the Dark Lord of the Prophecy.
Snape immediately felt that something had gone horribly wrong the moment he saw a section of the house where Lily Evans-Potter lived blasted apart. Holding out his wand, Snape cautiously entered the place and was immediately greeted by the lifeless form of his mortal enemy, James Potter. Even in death that ridiculous man looked arrogantly defiant. "And look at where that landed him, dead by the Dark Lord's wand," Snape thought.
The cloaked wizard stepped over the corpse to climb up the stairs. He immediately slumped to the floor as he saw Lily lying lifeless next to a crib that astoundingly held a live and seemingly unscathed child. The Dark Lord was nowhere to be found and yet, traces of Voldemort's magic were in that place; Snape could feel the skin under his tattoo tingling. "Could it be that the Prophecy had come true?" Snape thought fleetingly. The wizard crawled towards the woman's body and briefly cradled it in his arms. He was on the threshold of releasing tears but his mind was opened to a new and wonderful possibility. "Finally Lily, you will be mine."
Snape briefly examined the boy and saw a curious lightning bolt scar on the toddler's forehead. Electricity rushed up his left arm as he touched it. "This must be where the Avada Kedavra struck," he thought. Snape carried Harry Potter on his right arm and tightly held onto Lily's wrist and his wand using his left hand. He focused on his destination and Apparated to his house at Spinner's End.
Snape planned to bring Lily back to life using an unusual piece of magic that he had come across in an obscure book of the Dark Arts. He laid Lily's body and Harry in the middle of a ritual circle drawn on his home's floor, which he copied from the book. Listed in it, too, was a "recipe" of chemicals that made up the human body. Snape did not need to bother with that since Lily's body was right before him. The ceremony called for a sacrifice "of equal value" in order to call the soul of the dead person back. Her son was going to be that sacrifice.
The instructions in the book stated that the person holding the ceremony must focus all of his energy and channel it into the circle. When everything had been prepared, Snape focused on his happiest thoughts—that of Lily finally loving him back and the two of them living together—and cast a Patronus at the circle. The intricate patterns and words drawn inside the circle started to give off a bluish glow. Snape was beginning to feel elated by what was turning out to be a successful endeavor when suddenly the air inside the house became heavy and suffocating. A huge eye opened right inside the circle and fixed its gaze on Snape. Long, black, vine-like tendrils shot out of its pupil, encircling themselves on Snape's arms, legs, neck and trunk, and pulling him inside the circle. Snape tried to resist but his efforts were in vain. It was only when he gave in that he noticed that ends of the tendrils were actually tiny hands. He was sucked into oblivion.
After what seemed like eternity, Snape came face to face with a humanoid creature whose only recognizable feature on its face was its wide, grinning mouth.
"Who are you?" Snape asked.
"I am what you humans sometimes call the 'World' or the 'Universe' or the 'One' or the 'Truth'. But I am also really," the humanoid raised its hand to point an accusing finger at Snape, "You."
Snape turned to look at a huge door that materialized behind him. It opened to reveal the Eye once again. Like what happened earlier, the long back tendrils grabbed Snape. He was forced to travel through what seemed to be various dimensions at the speed of light. Billions of sounds and images flooded into the wizard's brain. The Truth's voice rang inside Severus' head, telling him that those who attempted "human transmutation" received a "gift" from it. He was now receiving all of the knowledge of the world.
"Stop it!" Snape screamed. "My head is going to burst!"
But the onslaught of stimuli did not cease. Just when Snape felt that he was going to die from a brain aneurysm, he saw Lily's figure reaching out to him. He suddenly realized that he needed to get to her to find out what he had to do to complete the resurrection process. "I'm about to succeed," he told himself as he reached out for Lily's hand which was lying outstretched towards him, "I just need a little bit more information."
He never was able to hold her.
Snape was slammed back to the place where he met the Truth earlier. "Please," he begged, "I'm so close to perfecting the resurrection! Please show me more!"
However, the Truth laughed at him and said, "I can only show you that much for that fee that you've paid."
"Fee?" Snape questioned.
"In exchange for all of that, I've extracted the magic out of you, of course," the Truth said.
Snape's face blanched.
"For you have just performed human transmutation—a forbidden branch of Alchemy, Mr. Ignorant," the Truth mocked. "And of course, in Alchemy, we must follow the Law of Equivalent Exchange."
While Snape had studied the basics of alchemy, he did not know anything about this Law.
"But the boy!" he protested, "The boy was meant to be the sacrifice!"
"And you think that a boy's life is equivalent to a soul?" the Truth countered.
Snape was silenced. This was Lily's soul, after all. In his arrogance he had forgotten how vividly she had lived. Of course her soul would have a great price on it.
"But you know, in all my years of existence, your attempt at human transmutation is probably the closest that anyone has come to what may be called a success," the Truth mused. "But that may be because of outside factors," it said, before grinning widely at Snape. "And to think that you didn't even know what you were doing!"
In less than the blink of an eye, Snape was back inside his house. Perhaps he had not left physically after all and it was only his consciousness that had travelled that great distance. A shuffling sound brought all of Snape's attention back into the present.
He was greeted by the sight of Lily getting up from the floor. At her feet was her dead baby. Well, that was a problem that could be easily solved by an Obliviate followed by some potions for inducing amnesia. It was then that Snape remembered that the Truth had taken away his magic.
"Severus, what have you done?" Lily asked.
Suddenly, Snape felt very, very tired. He would never be forgiven for what he had done, but maybe he could make her see that all that he did was because he loved her and could not live without her. He spilled everything—how he was torn by guilt for relaying the prophecy to Voldemort, and all the things he did to ensure that she would be protected when Voldemort came knocking upon the Potters' door. He told her of his wavering trust in Voldemort's and Dumbledore's ability to keep their promises. He told her of how devastated he was to see that the Dark Lord had not kept his word and killed her. It was then that he decided to take things into his own hands. He attempted to resurrect her using an obscure piece of magic. However, it required that something be sacrificed to get her soul back and that something was her son. He had planned to erase all of her memories of her time with Potter and the rift that had come between them but now, he couldn't do it because in exchange for his success in resurrecting her, his magic had been taking away from him.
After this confession, Lily laughed—a cold, high, unnatural sound that sent chills down Severus' spine. He was suddenly filled with dread. That kind of laughter… no, it couldn't possibly be.
"I never thought that you were this foolish Severus…" the woman in front of Snape said.
"No," Snape thought, "please let this all be a dream."
"…and yet, I must thank you for this foolishness because it has given me the chance to walk again…"
"Why? Why?" The question frantically echoed inside Severus Snape's mind.
"…in such a fine, young and rather interesting body."
There was suddenly a cold, hard glint in this woman's eyes, something never seen before in the eyes of Lily Evans, not even during the time she broke off her friendship with Snape.
"I should reward you, perhaps, with a kiss? You would like that, wouldn't you?" The woman moved closer to Snape who, on the other hand couldn't move a muscle. He must have been hit with a wordless binding spell. Her hand moved up to touch Severus' cheek. He closed his eyes. The kiss never came. He opened his eyes again to find his own wand pointing at him.
"I have no use for a traitor and Squib like you. Goodbye Severus. AVADA KEDAVRA."
Author's Notes:
I hope I was able to write the story clearly enough so that those who haven't seen the movie or the anime may understand what's going on. However, I'll still be posting a few explanations below because there are concepts that I couldn't weave into the story without it turning into a tedious mess.
In the story, Snape unwittingly uses obscure magic, Alchemy, which is a different from the alchemy that the Wizarding World knows. (Note the capital "A" and small "a" used in the story.)
The source for this Alchemy is the Full Metal Alchemist universe. The events surrounding Snape's resurrection ritual are very much like the events surrounding the anime's protagonists' failed attempt to resurrect their dead mother.
The Law of Equivalent Exchange that the Truth mentioned states that one cannot gain something out of nothing; if one wishes to gain something then one must present something of equal value in return.
The Truth's question, "And you think that a boy's life is equivalent to a soul?" is just rhetoric but Snape thought that the Truth was talking about Lily's soul, which was impossible, right from the start, to recall. The ceremony then proceeded to extract the extra piece of soul that was lying inside the ritual circle, Voldemort's horcrux. The effort was too much for young Harry's body to bear which was why he died. Harry's life was not used in exchange for anything.
The extraction of Voldemort's fragmented soul from Harry's scar and its attachment to Lily's body was what Snape got in exchange for the loss of his magic. We now have a Voldemort walking around in Lily's body. Creepy.