Chant of Vitu-Ghazi
Instant
Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for 
or one mana of that creature's color.)
Prevent all damage that would be dealt by creatures this turn. You gain life equal to the damage prevented this way.
Illustrated by
March of the Machine Commander#177UncommonEnglishLegality
Rulings
The damage prevention shield lasts for the rest of the turn, and you gain life each time that shield prevents 1 or more damage.
The rules for convoke have changed slightly since it last appeared in an expansion. Previously, convoke reduced the cost to cast a spell. Under current rules, you tap creatures at the same time you pay the spell’s costs. Tapping a creature this way is simply another way to pay.
Convoke doesn’t change a spell’s mana cost or converted mana cost.
When calculating a spell’s total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated.
If a creature you control has a mana ability with 
in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped when you pay the spell’s costs. You won’t be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won’t be on the battlefield when you pay the spell’s costs, so you won’t be able to tap it for convoke.
Tapping a multicolored creature using convoke will pay for or one mana of your choice of any of that creature’s colors.
When using convoke to cast a spell with 
in its mana cost, first choose the value for X. That choice, plus any cost increases or decreases, will determine the spell’s total cost. Then you can tap creatures you control to help pay that cost. For example, if you cast Chord of Calling (a spell with convoke and mana cost 
) and choose X to be 3, the total cost is 
. If you tap two green creatures and two red creatures, you’ll have to pay .
Because convoke isn’t an alternative cost, it can be used in conjunction with alternative costs.