Y'vein, the Sealed Fate

If a player has sacrificed six or more creatures
this turn, you may cast Y'vein, the Sealed Fate
for {b}{b}{b} from any zone as though it had flash.
At the beginning your upkeep, each player sacrifices all creatures they control and creates an
X/X black horror creature token with fear,
shroud, and "This creature cannot be sacrificed."
where X is equal to the total power and
toughness of each creature that player
sacrificed this way.