Necromantic Arts NEWT

Harry always finds a way to get into trouble, even when he isn't trying. Well, he was certainly not trying to do anything but take his NEWTs and become a productive member of society after the war with Voldemort was won. But what is this about a Necromancy NEWT and how did he come to take it? Better yet why is there a dead cat now sleeping in his bed.
Only Death knows.