Harry Potter and the Soul-Mate Ceremony

As the Aurors arrived they found a quiet house full of the smell of candle wax and cats. They searched every room until they found what they where looking for. The old lady was sitting in a chair, dead. And on the desk in front of her was a piece of parchment paper, on this paper was written a prophecy. This prophecy would either be the end of everything or the beginning of a golden age. They searched the area for signs of dark magic, but knew they would find none. For this elderly witch had died peacefully in a trance. After they had checked everything they let the Mediwizards and the other Ministry officials to clean up and make the area safe for Muggles. But before they left they grabbed the prophecy and put it in the archives, hoping that somewhere in the future it would be of use to someone. For the prophecy told how the greatest evil of the world would either be crushed or helped by two seemingly unordinary wizards. This is what it read:

In the time to come when evil is abound. A wizard that was will rise to claim us all will be crushed by powerful spell of love. He will not be killed but be in exile until he is able to rise again. When this comes to pass nothing will stand in his way, except a witch and wizard who's power is above us all. Their love will either conquer this dark lord, bringing about a golden age or they will usher in a reign of terror like none before.

So this is how the Soul-mate Ceremony started in the community of wizards. This ceremony binds two people together for the greater good so that if the dark one ever appears there is a weapon against him. But this same weapon could be turned and used for the darkness so they must both be of a pure heart and soul.