I do not own Harry Potter, J.K. Rowling does. All I own is the changes I made in my head, and put into words here.
It was a perfectly fitting Halloween night - dark, and while it wasn't raining there were definite thunderclaps in the distance.
For Lily Potter, currently of Godric's Hollow, these thunderclaps meant more than just atmosphere. It meant that, on the one hand, her house could be entered without her ever knowing - but on the other hand, it meant that the spells and potions she'd designed would definitely work even if she had to do the final steps alone.
Months ago, when Severus Snape unexpectedly contacted the Potters, a slight panic had ensued. For one, it was immediately told that Peter Pettigrew, their secret-keeper, had squeaked "like the rat he played at being". The Fidelius Charm had fallen that night, and hasty protections erected to stave off the inevitable.
Mercifully, Snape then told them that Voldemort's plans were to attack on Halloween - when it would be easiest to subvert prophecy. This was good. This gave them time to prepare, to maybe survive. Escape plans were made and discarded - Voldemort would be watching the cottage now, and with his spies in the Ministry he'd even know if the Floo was used. There was no safe way out, and the Potters knew it.
Eventually, a viable plan was made. Lily, with help of Albus Dumbledore, had set her talents in Charms and Potions to work, creating a combination - a ritual, though the Wizarding World didn't use that word often - that might work. Albus Dumbledore, with his political connections, also made good on a promise to let Severus Snape live in exile - preparations were made for the man to change names and move out onto the continent, far away from any reminders of his homeland.
Felix Felicis, or a version thereof, played a large role - while ordinarily the potion was decidedly unsafe for children, its effects were ideal for the circumstance. Magic, a slightly alive force, would not take kindly to the attempt to subvert prophecy - and Felix Felicis was a potion that let Magic "take the wheel".
The potion, combined with saturating the ambient forces of magic and several spells layered on Harry Potter himself, were all meant to strengthen the prophecy - specifically, "but he shall have power the Dark Lord knows not". A spell was created to strengthen Harry's accidental magic, and then a variation of Felix Felicis would let it guide Harry instead of the other way around.
It would only last for a few moments, and given the unpredictability of accidental magic, it was a scary prospect. But, it was the best they had.
Lily took a few moments to comfort Harry. James was still out to fetch Albus - it would be easier and less risky for all three of them to work the spells, and to contain any unforeseen effects. In a few short moments, Lily would have to do all of that alone, and the spells would cause Harry some degree of distress as they affected the baby's senses.
All too soon, the clock struck - and predictably, the sound of thunderclaps became a loud banging on the magically-sealed door. There was no more time.
Muttering incantations, Lily kept her son in as tight a hug as she could, fueling the magic with her love. Just as a loud bang rocked the house, Lily finished the last spell, and quickly produced a baby-bottle laced with the altered Felix Felicis - Harry drank without noticing anything odd, at least for now. Just as Lily placed Harry back in his crib, the door to the nursery opened.
The last thing Lily saw was a flash of red - Severus had apparently made good on his attempt to get Voldemort to spare her, a last apology by her former friend, accepted by the Dark Lord under the guise of him wanting Lily for himself - before unconsciousness took her.
Harry sensed, rather than saw, the stillness in his mummy - and the decidedly malevolent energy from the figure at the door. In his childish mind, a lot of things suddenly felt wrong - his Mummy was too still on the floor, he didn't know whomever this was, and the energy he was emanating was so foul it was hurting Harry just to be near it.
A sharp shock prickled the toddler, who responded instinctively by standing up. Just as he heard a strange word, "Avada -", something shifted. Harry knew that he was doing something, but didn't know how to control it - even as it blew the strange man across the room repeatedly, the pale green light from his magic vanishing as the stick - no, wand - was pulled violently from his hand.
Harry vaguely realized he wasn't actually in his crib anymore. He was floating above it, still held in the throes of whatever this energy was. The man had long since gone even more still than mum - he wasn't even breathing anymore, whereas mummy still was.
At that moment, James Potter and Albus Dumbledore returned. Shocked, they both watched as Harry Potter floated above his ruined crib, everything else in the room whirling dangerously - Harry himself seeming to be as distressed as they felt, as occasionally a flash of light caused him to jerk around.
At that moment, Harry felt an intense double heartbeat. His father and the kindly old man who sometimes visited, seemed to have regained their senses just in time to witness this last surprise - Harry saw a vision of a small bird he didn't quite recognize, but that looked kind of like a small version of the birds that sometimes brought his mum and dad things. At that moment, he wanted nothing more to be not hurting his mummy and daddy, to not be Harry Potter who was currently making a mess of the nursery. His mind latched onto becoming that bird he just saw. Not Harry, the baby who was causing his nursery to break - but Harry, the little bird in his nest.
As the whirlwind of energy and objects subsided, Albus and James were in for one last surprise, as little Harry Potter, all of one-and-a-half year old, did what they would never have predicted. Just as Lily woke up, she too watched as her little baby boy completed the Animagus transformation, a little gray-white owl chick laying exhausted and asleep in the ruins of what once was his nursery room.