Buy @ TCGplayer
When Miasmic Mummy enters the battlefield, each player discards a card.
When Miasmic Mummy’s triggered ability resolves, first the player whose turn it is chooses a card to discard, then each other player in turn order chooses a card to discard, then all of those cards are discarded simultaneously. No one sees what the other players are discarding before deciding which card to discard.
