If a spell or ability an opponent controls states that something happens unless you sacrifice a permanent (as Mogis, God of Slaughter does) or discard a card (as Painful Quandary does), you can’t choose to sacrifice or discard. On the other hand, if a spell or ability an opponent controls instructs you to sacrifice a permanent unless you perform an action (as Killing Wave does) or discard a card unless you perform an action, you can choose whether or not to perform the action. If you don’t perform the action, nothing happens, since the spell or ability can’t cause you to sacrifice any permanents or discard cards.