Miasmic Mummy

• WWizards of the Coast2017-04-18
  Official Ruling
  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.

Rulings via Scryfall