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At the beginning of each player's upkeep, that player sacrifices a non-Zombie creature.
At the beginning of the end step, if no creatures are on the battlefield, sacrifice Call to the Grave.
If a player controls no creatures, or each creature they control is a Zombie, that player does nothing when the first ability resolves.
If there is at least one creature on the battlefield at the beginning of the end step, the last ability won’t trigger. If it does trigger, but a creature enters the battlefield before it resolves, the ability won’t do anything when it resolves. Call to the Grave’s controller won’t sacrifice it.
In a Two-Headed Giant game, Call to the Grave will trigger twice during each team’s upkeep, once for each player. Each of the two players will sacrifice a non-Zombie creature when the ability referring to that player resolves.
