Protection: The Sword and the Shield

Smallest Bedroom

Blood magic is not truly forbidden. Not even in the highly conservative British wizarding society. Instead it is highly regulated. A mundane equivalent would be nuclear technology. Like nuclear technology, the uses and dangers of blood magic are vast and not fully understood. Because of this, legal practitioners of Blood magic submit themselves to a great deal of governmental regulation and oversite.

James and Lily Potter were magical prodigies and whileneither worked in the field of Blood Magics, they both had a remarkable understanding of it in addition to other magics. So when it became clear that their son was a target of the worst Dark wizard in a millennia, they both devoted their intellects toward protecting their most prized achievement.

In the end, what Lily and James Potter did to protect their son was the equivalent of two very brilliant physicists building a thermonuclear bomb in their basement. To complete the analogy, the complexity of the particular blood magic ritual that the Potter's created and implemented would be the equivalent of those two physicists building a nuclear bomb in their basement and that bomb having all the yield of Hiroshima and Nagasaki combined with no radioactive fallout. Basically, the two magicals created what was previously thought impossible.

On arrival after the tragedy, Albus Dumbledore, himself considered the greatest and most knowledgeable magic user of the past 2 ages, was able to determine the result but unable to decipher how it was done. The only thing he was sure about was that one of the deathly hallows had been used as part of the ritual. The result was that Harry Potter's blood was granted a protection that was tied directly into his father's legacy and his mother's love.

The great wizard could not figure out how to use the Potter Legacy but was able to find that the home of anyone with Lily's blood could power near impregnable wards. Acting on this he quickly made arrangements and secreted Harry Potter off to his aunt's home.

James and Lily did not aim to protect their son's home, they aimed to protect their son and had Albus Dumbledore left their efforts alone the power of that protection would have ejected the leach that was currently attached to his forehead. Instead Albus Dumbledore had diverted some of the Potter's efforts toward warding his home leaving just enough power to contain but not eradicate the soul of the most evil and powerful dark lord in centuries.

......Smallest Bedroom, 4 Privet Dr., Surrey, England

Sleeping on his bed in Petunia's home, Harry Potter began to glow. The foreign soul now stood out in Harry's mind and magic due to his semi-accidental occlumency and the power of the protection ritual recognized it as the greatest threat to its charge. Before it had been focused on protecting Harry from his relatives which was why his abuse never became as physical as the Dursleys might have wanted.

Now, a combination of Harry's new life plus the clarity of the magical infection against his organize mind caused the protection to break from Dumbledore's meddling, collapsing the houses ward, and put the full force of that protective magic to use against the soul fragment.

Harry's scar burst open while he slept peacefully in a magically induced coma. Part of the original ritual's magic was designed to use the attacker's power against it and the soul fragment had much of Voldemort's power still in it, much more than was needed to destroy it. The ritual was designed to feed any excess power into the Potter legacy. If Harry was in a Potter Property or maybe even in a magical area the excess magic would have gone to a Potter property's wards or to a house-elf. But living in muggle England meant that Harry himself was the only example of the Potter Legacy anywhere close.

Protective magic cannot destroy what it is meant to protect and so the excess magically activated all of the latent magical abilities that existed within Harry's DNA, strengthened and increased his magical core, and ripped all the magical knowledge from the dying soul and dumped it into Harry's mind. With the last bit of excess magic the ritual anchored itself to Harry's mind and core in addition to his blood.