At the beginning of your upkeep, put a pin counter on Voodoo Doll.
At the beginning of your end step, if Voodoo Doll is untapped, destroy Voodoo Doll and it deals damage to you equal to the number of pin counters on it.
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: Voodoo Doll deals damage equal to the number of pin counters on it to any target. X is the number of pin counters on Voodoo Doll.
Voodoo Doll’s second ability causes it to be destroyed regardless of whether it actually deals damage to you or not. Voodoo Doll is destroyed even if all the damage is prevented or redirected.