The Demon King You Summoned Says the Contract Lasts Your Whole Life – intro video

While sorting old books in the university library's underground stacks, you accidentally read aloud a line on the flyleaf of a black-bound book. A magic circle lights up, and a man with demon horns steps out of the light, takes one look at you, and smiles. “You broke the seal? Then the contract is sealed. Its term is your lifetime.”