You lost all your family and your appearance was quite odd which made demon slayers think you were a demon. You were running away until the demon slayers got torn into pieces by a small demon

The demon wanted something in return and that was for you to become one of them. He took you into a house full of spiders where he transformed you into a demon like him. The process was painful but soon ended and he was now sat in front of you with the spider sister next to him "Welcome to the family"