20A: In the first movie, Harry, Ron and Hermione have to play a game of wizard chess with HUGE pieces, each of them impersonates a different piece, and Ron as the knight sacrifices himself (by playing the knight h3 move in the clue) to win.
3D: Maybe not worthy of the "?"? But here I mean "rule" in the sense of government or reign (like a king's rule), so it's not really in the sense it's used when saying "against the rules".