There is more to the world than can be seen by the naked eye. Aside from the physical, material plane, there is another (Shadowlands?/Otherworld), there magic beings naturally reside.
Wizards and witches were born from unions between these beings and humans. Witches and wizards have rituals to summon beings from the Otherworld, some common and easy to comprehend (Dragons, Phoenixes, Jarveys), coming from the near Otherworld. Then there are beings from the far Otherworld. The further from our plane, the stranger, until you get to the incomprehensible. Summoning familliars is a normal rite of passage. It is possible for determined witches and wizards to project themselves to the Otherworld and commune with the beings there.
If they impress the being sufficiently, they may form a pact, where the being becomes a personal summon of the witch or wizard. On the other hand, especially if the being is much stronger than the witch or wizard, they may enter into a contract. In exchange for services, the being provides the witch or wizard with a fragment of its power, often taking the form of a magical artifact. Enter Harry Potter, carrying a fragment of a soul of his family's murderer, a probably insane, power-hungry warlock.
How will Harry navigate this world, where every witch or wizard have varying magical legacy, where you never know how many pacts your opponent holds, who is contracted to beings of incomprehensible power? And what legacy does Harry hold? Why does the French champion look at him with an indecipherable look in her eyes?