2017-11-17 : After you discard your hand and the number of creatures each opponent must sacrifice has been determined, each opponent in turn order, starting with the player whose turn it is if that player is your opponent, chooses which creatures they will sacrifice, then all the chosen creatures are all sacrificed at the same time.

2017-11-17 : Malfegor’s ability is not optional. You must discard your entire hand no matter how many creatures your opponents control.