While resolving Rottenmouth Viper’s triggered ability, your opponent chooses a card to be discarded without revealing it, chooses a nonland permanent to be sacrificed, or chooses to do neither. Cards or permanents they’ve chosen previously during this process can’t be chosen again. Then that player repeats that process if it hasn’t yet been done once for each blight counter on Rottenmouth Viper. Finally, that player discards the chosen cards, sacrifices the chosen permanents, and loses 4 life the appropriate number of times.