Opera Masquerade

When Opera Masquerade enters the battlefield, exile
all nontoken face-up creatures face down. Starting
with you, each player chooses a random card exiled
this way and puts it onto the battlefield under their control face down. Repeat this process until all cards exiled this way have been chosen. (They're 2/2 creatures.)

At the beginning of each player's upkeep, that
player may turn a face-down creature they control
face up.