you were being chased by demon slayers and you ran up to Mount Natagumo and you ran into Rui as he stands on a huge spider web, he sees you tumble to the ground and the demon slayers pull out there swords and beat you to pulp. He kills the demon slayers with his webs, and walks up to you, he tells you, you're apart of his spider "family". Later that day he gets a bowl and puts his blood in it

"drink, and you'll become a demon like all of us. Then I'll decide on your role in my family.."