Catrimorph

A father and son lived in an old house. His mother had died, and his father was constantly drunk. To help his father quit drinking once and for all, the son began practicing black magic and shamanism. And then one day, he succeeded, albeit at a high price. He summoned a demon through a ritual, but due to his inexperience in conversation, he made a very unfavorable deal: the demon made sure his father never drank again, but the son forgot to mention alcohol. Ultimately, his father died of dehydration, and the son (who had chosen not to need money) became its slave, bound to exist in the house for eternity and evoke the demon's fear from the residents, forever remaining in the guise of a monster under the bed. However, there was a bonus for the boy: he had the power to transform into any animal or object. For this ability, the demon gave him a new name—Catrimorph.

However, when a family moved into the house, Katrimorph suddenly undermined his master's trust: he became friends with the younger daughter and fell in love with the older one. And harassing the parents wasn't an option, as that would have meant the immediate relocation of the tenants. However, when Sonya, the youngest, told her parents about her "new friend" at dinner, the father flatly refused to believe it. Meanwhile, the mother, being a believer, began to wonder...