Two fic searches
1.) Ron dares Harry to buy an adult toy. Harry buys the toy and when he uses it he is transported to Lucius Malfoy's bedroom.
2.) Harry is waiting to go to Hogwarts at Kings Crossing when a naked Lucius Malfoy falls into his lap. I think he was tortured by Voldemort. He tries to seduce Harry and Harry accidentally cuts himself. I remember that it happened in front of Snape.