Disclaimer: I have no money and I own nothing. Harry Potter and his world belong to JK Rowling.
Chapter 1: Goodbye
Inside of Hogwarts School of Witchcraft and Wizardry, Severus Snape watched his son finish preparing the runic inscriptions on the floor of the office in which they were currently barricaded, well aware that this would likely be the last time he ever saw him.
As time was now of the essence, Severus pushed his distress aside, and he quickly hurried over to Minerva McGonagall, the current and probably final Head of Hogwarts, who was lying on a tartan covered couch.
"You understand what will happen if this succeeds?"
No longer able to speak, Minerva simply nodded and grasped Severus' hand. She then squeezed it in both a confirmatory and comforting gesture.
After sparing Minerva a smile, Severus turned to his son, who was standing at the edge of the runic circle he had just finished drawing.
"Father, you're the best thing that ever happened to me, and I…" The boy's voice trailed off as he became too choked up with emotion to continue.
Severus pulled his son into his arms, not ever wanting to let him go. Unfortunately a yell from outside of the door alerted Severus to the fact that his feelings were going to have to be put aside, as the time to try the experiment was finally upon them. He therefore placed a kiss on the boy's forehead and let him go.
"I love you too, my son."
After being released, Severus' son stepped swiftly into the circle, drew his wand, and began the incantation to set in motion a ritual that, if successful, would give them a fighting chance to destroy the wizarding world's most feared adversary, Lord Voldemort.
As the final word of the ritual was uttered, Voldemort broke through the last ward Severus had placed on the door, and he entered the room just as Severus' son vanished, letting out a scream of frustration as he spotted the runic inscriptions on the floor.
However, he had time to do little more than scream because the magical backlash from the ritual spread out like a tidal wave, and, in less than the blink of an eye, everything with a two mile radius of Hogwarts was laid to waste.
Severus's final thought was 'we did it' before the darkness took him.
His son's first thought after the transfer was 'oh shit' as he stumbled towards a toddler who was sitting on a mat in front of him. However it was not his tumble towards the child that had caused the rather impolite reflection, but the fact that a bolt of green light was heading directly for them.
His final thought before darkness took him, as it had his father, was 'we've failed'. He had no idea that they had done nothing of the sort, but the world he had just appeared in was about to pay a hefty price as his arrival sent tidal waves of time rippling across it, forever changing how its history and future would play out.
Note: Credit to Madnesspersonified for the similar story title.