If I have three creatures that each have a vigilance counter on them, can I activate Tayam, Luminous Enigma by removing their vigilance counters and sacrificing them to Phyrexian Altar?
1 Answer
601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs.
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601.2g If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, “Mana Abilities”). Mana abilities must be activated before costs are paid.
That means you have to activate Phyrexian Altar before paying the costs, and removing the counters is part of the costs. So what you propose won't work.
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I'd note that you can sacrifice a creature in response to his ability being put onto the stack, then return them back onto the battlefield when his ability resolves, however. May 6 at 13:33