Disclaimer for the whole story: I do not own Harry Potter JK Rowling does!
Prologue
Godric's hollow, Halloween 1998
Mooney and Padfoot didn't baby-sit very often but they were tonight. They'd practically forced Prongs and Lily out; they had been very stressed lately. Sirius had paid for the outing; Merlin knows his family could afford it.
The Twins were upstairs in their nursery asleep, in their cots. Padfoot and Mooney were in the lounge, playing a game of wizard's chess.
The clock on the mantle piece reached midnight. A red flash filled the room and both men slumped forwards unconscious.
Lord Voldemort walked upstairs, wand out, an abnormally large python coiled around his body. He opened the door of the Twins room. Both children were awake now, having sensed that something was wrong. Carved on the cots were names on one it said Jack and on the other it said Harry.
Voldemort approached the cribs. One of the children sat up and gazed at him, with eyes the colour of the killing curse. He settled his wand on the hazel eyed child, who was watching his brother trustingly. The slightly older, green eyed boy, Harry, glared at the sensed threat to his brother.
Suddenly the curtains behind Harry spontaneously caught alight. The fire started to eat away at the fabric, smoke filled the room as the flames spread. The snake started to hiss violently. Voldemort's wand began to char slightly, Harry was gazing narrow eyed at the wand.
Voldemort turned to Harry and said "Interesting. An elemental, rare yes, but a power I don't know I think not." He levelled his weapon at the boy and uttered the lethal words "Avada Kadava" a green light burst from the wand and headed straight for the boy.
However, instead of killing the target as it was designed to do, the curse was absorbed into the child, depleting the casters magical core and a cut formed on the child's hip as if an invisible knife was slicing through the babes flesh leaving a cut in the shape of a lightning bolt.
Panicking as his magic left his body, Lord Voldemort cast a desperate and feeble "sectumsempra" at Jack leaving a V shaped cut at the base of the child's neck. This was Voldemort's last act before his shattered soul was purged from his body, which faded from the room along with his wand, leaving only his clothes behind. The snake now on the floor as its master had disappeared, slithered away and out of the house through an open bathroom window.
When Padfoot and Mooney came around they sprinted up stairs and burst into the nursery. They froze in shock at the scene before them.
The curtains and tapestry were ablaze, spewing smoke into the room at an alarming rate. In the centre of the room was a pile of adult's clothes, no sign of their owner. The Twins were sitting, staring at the clothes, perplexed, but still alive.
After that night Jack Potter was proclaimed as the boy-who-lived by Albus Dumbledore and Harry the slightly older twin slipped into the background.