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It's about a door that leads to two worlds the human world and the demon world and a demon comes out the door hoping to take over the human world but after meeting a human and getting to know her while in a human disguise has made him fall madly in love with her but when she denies him of becoming a demon too he gets mad and goes back to his original idea and then she makes him a deal that if he spares the world she'll become a demon too but in the end he let's her stay as a human and he turns into one.