For this one I’m going for Horcruxes mainly because I think its brilliant that Voldemort split his soul into so many pieces. Which everyone thought was so impossible. For Dumbledore to then start finding them and explaining to Harry that they must all be destroyed before Voldemort can die.

How manipulative each horcrux can be, with the diary possessing Ginny Weasley and the necklace making Harry/Ron and Hermione see very strange things if they wore it for too long. Just showing how powerful Voldemort really is.