When Malfegor enters the battlefield, discard your hand. Each opponent sacrifices a creature for each card discarded this way.
11/17/2017 Malfegor’s ability is not optional. You must discard your entire hand no matter how many creatures your opponents control.
11/17/2017 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.