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At the beginning of your upkeep, each player sacrifices a non-Vampire creature of their choice.
If a player controls no creatures, or if all creatures a player controls are Vampires, that player simply doesn't sacrifice anything.
When the triggered ability resolves, first you choose which creature you'll sacrifice (if you control any non-Vampire creatures), then each other player in turn order does the same, then all chosen creatures are sacrificed at the same time.
