702.51. Convoke

Convoke is a static ability that functions while the spell with convoke is on the stack. “Convoke” means “For each colored mana in this spell’s total cost, you may tap an untapped creature of that color you control rather than pay that mana. For each generic mana in this spell’s total cost, you may tap an untapped creature you control rather than pay that mana.”

The convoke ability isn’t an additional or alternative cost and applies only after the total cost of the spell with convoke is determined.

Example: Heartless Summoning says, in part, “Creature spells you cast cost {2} less to cast.” You control Heartless Summoning and cast Siege Wurm, a spell with convoke that costs {5}{G}{G}. The total cost to cast Siege Wurm is {3}{G}{G}. After activating mana abilities, you pay that total cost. You may tap up to two green creatures and up to three other creatures to pay that cost, and the remainder is paid with mana.

A creature tapped to pay for mana in a spell’s total cost this way is said to have “convoked” that spell.

Multiple instances of convoke on the same spell are redundant.