One possible 'brother in law hypno black demon story' could be that the brother - in - law is an amateur magician who dabbles in hypnosis. One day, while practicing in an old abandoned house, he accidentally summons a black - demon - like entity. The demon starts to play tricks on his mind, making him see strange illusions. He has to use his knowledge of hypnosis to fight back against the demon's influence and break free from the terrifying situation.
I'm not sure there are specific well - known 'brother in law hypno black demon stories' in general. It could be some very niche or even made - up concepts. Maybe it could be about a brother - in - law who gets involved in some strange hypnosis - related events and there's a black - demon - like figure in the story, but without more context, it's hard to tell a specific story.
There could be a story where the brother - in - law is a curious researcher. He discovers an ancient book on hypnosis and black magic. Ignoring the warnings, he tries out one of the hypnosis spells from the book. Unbeknownst to him, it awakens a black demon. The demon then starts to haunt him, getting into his dreams and making his life a living hell. He then has to find a way to reverse the spell and banish the demon with the help of a local wise man.