One of the darkest and most infamous aspects of demonology would be the concept of making a deal with a demon. In my story about Hell, the main character ends up in Hell not because of a particular sin. Instead, he was sent to Hell due to unwittingly making a pact with a demon. The demon in question is a young succubus (female demon) who desires the protagonist. In exchange for making the protagonist’s ultimate ambition come true, the succubus gets him upon his physical death. The way they perform the pact is somewhat unorthodox. Over time, the relationship between these two characters evolves to the point in which they genuinely love each other. You could say that their story will be one of forbidden romance between a human and a demon.