Very AU. There was no separation between muggles and magicals. Wands were never invented, instead, witches and wizards shaped their raw energy to reach the desired effect. Especially in wars, rulers coveted magicals for their strange powers. Later in history, a famous magus discovered the power of foci. Usually in the form of jewels, these allowed a magical to connect his magic to an element. While all magicals could control every element if they had the right foci on hand, they all also found one element to easier obey their will. This transformed not only warfare, but everyday life as well. Magicals soon after discovered that they could form a pact with another magical being. When that being died, they could store their soul into a special focus. This allowed them to call on that soul and craft it a new, temporary body, of an element of choice. Again, life changed, magical summoners dominated the battlefields.
Another discovery was made, one which marked the beginning of a catastrophe. Our world was not the only plane of existence. An enterprising magus managed to create a gateway between our world and another plane of existence, known as the Nether Realm. The inhabitants of the Nether Realm, varied as they were, had one desire all in common. They wanted to experience our world. Yet, they were beings of energy and to experience our world, they needed our senses. They fed on magic. They could consume the entire magical aura of any being and thus possess them. Or they could consume only part and leave a part of themselves behind, thus tainting the original being. This Nether remnant grows slower over time, until a new Nether Demon is formed. It then possesses its original host. Nether summoners became the scourge of battlefields, a few well-placed Nether Gates could cause the collapse of a city.
To counter this immense threat, magicals created a weapon, special crystals that could be used to store Nether energy. If they could catch a Nether Demon without a host, they can seal the Demon into the crystal. A special purification ritual is then used to dispose of the sealed Demon.Although non-magical people were of no interest to demons, the magicals needed protection still. A ritual of controlled seeding was created when an extremely weakened Demon would be summoned, allowed to attach itself briefly to a magical and then be disposed of. This would leave a small, extremely weak taint in the magical. Nether Demons, it turned out, do not target already tainted beings. Such a weak seed would either never sprout, or, if it did, it would be easily crushed, leaving the original host pure. Enter Voldemort, a magus of prodigal power who pushed the boundaries of magic further than anyone alive. Guided by a prophecy he attacks the Potter family, both parents fall, yet he fails to kill their son. Harry doesn’t come out of this encounter unscathed, however. His skin has always a deathly pallor, his eyes glow red, even non-magicals can feel something unnatural and unsettling about him. He feels little to no pain, his emotions are dulled. Harry bears an extremely heavy tainting, wielding a mix of magic and Nether Energy. Those who were friends of his parents dread the day when the seed within him sprouts, for surely the resulting Demon will be extremely powerful.
Enter Fleur Delacour, pale, silver-haired Templar, an expert at fighting Demons, wielding her flames and blade with equal skill, she detests Demons, having lost many friends to them. Yet Fate has brought her together with Harry Potter, possibly a walking disaster waiting to happen, yet also a young man who repeatedly thwarted the plans of the most insane magus of the recent centuries.