On the day of his death, I had already finalized the second part of the plan. I decided to involve that cowardly Trevor Nott. I cast the Imperius Curse on him and told him he had to visit his grandfather in Azkaban to confuse him. It was Trevor, under my order, who planted the story that you also know. Nott Senior had to believe he came up with the idea himself, to exchange his grandson and use him as a pawn to carry out his old revenge against the Malfoys. Mind you, his grandfather seriously intended to kill Scorpius, but he was too weak and elderly to do it without any help from us. That's why it was so easy to confuse him, even with the help of a mediocre wizard like Trevor.