Professor Vampire.

Dumbledore always believed Voldemort was the greatest threat to the wizarding world until a thousand-year-old vampire showed up at Hogwarts with the deed in hand—

“A thousand years have passed, and Hogwarts' lease is up!” said Dracula. “Headmaster, you wouldn't want your students to be homeless, would you?”

“What do you want?” Dumbledore asked gravely.

“Anything interesting.”

“Well, I think the position of Defense Against the Dark Arts professor is quite interesting!”

...

And so, Hogwarts gained a professor who couldn’t be dismissed.

McGonagall: “Albus, I don't think Professor Dracula's teaching is suitable for the poor children!”

Dumbledore: “There's nothing I can do, no one can fire him.”

...

Draco: “My father is a governor, I'll have him get you fired!”

Lucius slapped Draco on the head, “Mr. Dracula, Draco speaks without thinking. Please, don't fire the Board of Governors!”

...

This is the story of a vampire who lived too long and found amusement in the wizarding world out of sheer boredom.