Disclaimer: If I owned either Harry Potter or Yu Yu Hakusho I wouldn't have to work two jobs to pay for college.
Prologue
The dark circle pulsed with power as they chanted. A thrill of excitement ran through him, it was working. The chant sped up as swirls of energy, black and dark red, began to emanate from the symbols painted on the ground. It had taken years of searching, waiting, avoiding the Aurors' attention, all the while hiding the vengeance he felt for the death of his lord; but it was finally time.
A sudden wave of fiery light erupted from the summoning circle, those surrounding it flinched back and the chant faltered, then stopped completely as they stared in disbelief. The light vanished, and in the center of the circle stood a demon. It was easily seven feet tall, with grey leathery skin, several horns protruding from it's head, and deadly looking fangs and claws. The demon turned toward him with a snarl, the wizard suppressed a yelp and pulled out the talisman he had spent so long searching for. As the demon approached he quickly chanted the activating spell, then watched in awe as it took effect. The demon stopped its advance then stepped forward and bowed before him.
The wizard smiled triumphantly, "Rise demon," he commanded, it obeyed.
"You will be the first of many," he said, "you and your kind will serve us in our quest to cleanse this world and restore its rightful order."
The witches and wizards surrounding him cheered and he felt confidence sweep through him. They would avenge their lord and fulfill his work; with this new-found power, no one could stop them.
Author's note
So this is my first attempt at writing a fanfiction, if anyone has advice I'd love the to hear it.
This takes place after the 7th harry potter book, but before Harry and Ginny get married. In this story Harry's going through Auror training, I have no idea if there is cannon for this so if you know, please let me know otherwise I'll mostly be making it up.
This is after the end of Yu Yu Hakusho, but only a few years cause Genkai is still alive.