Convoke is a static ability that functions while the spell is on the stack. “Convoke” means “As an additional cost to play this spell, you may tap any number of untapped creatures you control. Each creature tapped this way reduces the cost to play this spell by {1} or by one mana of any of that creature’s colors.” Using the convoke ability follows the rules for paying additional costs in rules 409.1b and 409.1f-h. It can’t reduce the cost to play a spell to less than 0. See rule 502.46, “Convoke.”Convoke