HP×Underworld xover, Harry is a member of The Order, a secret society of Magical humans that exists to protect muggle society from the war between vampire and lycanthropes. Unsure of his convictions, and adverse to violence in general, his world is changed when he stumbles upon a resurrection ritual to bring back one of the ancient original vampires. Used as a blood sacrifice, because simple human blood won't suffice, he finds him self enthralled to the newly reborn Madam Delacour.
The vampires that brought her back expected a powerful weapon against the werewolves, a vampire matriarch they could use to bring down the hybrids, what they got was something new entirely. Something that started with her first servant Harry, which they could not join for they lacked the magic blood of the the muggle Order. The war wasn't coming to an end, it was changing fronts, and their only hope was to bring down the Delacours first and most favored general before they could create more.