Sometimes the villain in a work hits much closer to home than the hero would expect - it's their parent. Yes, the parent who we've seen throughout the movie, book or show, doing their usual parent stuff and being seemingly a good guy, only to turn out to be evil all along. Can be very shocking if pulled off right. This also refers to when a character's previously unseen parent is revealed to be evil. Not the same as Luke, I Am Your Father, which is basically the inverse - the villain (who was previously shown) is suddenly revealed to be the parent, rather than the other way around.