Convoke

Convoke is a keyword ability associated with the Selesnya guild from Ravnica: City of Guilds. The mechanic was originally devised by Richard Garfield.

Description
Convoke works by allowing a player to tap creatures rather than pay some of the mana cost of a spell. Each creature that is tapped pays for either or one mana of one of the creature's colors. By nature, this is a green ability, an even quicker way of paying for spells as green is by nature quick; though as the Selesnya guild mechanic it is also found in white. Nonetheless, in Magic 2015 there are convoke spells of all colors and none.

After its original appearance, Convoke was a returning mechanic in Future Sight, Magic 2015, Guilds of Ravnica and Modern Horizons.

Originally, convoke was an additional cost that reduced the cost in mana. With the return of convoke in Magic 2015, the rules for convoke changed to their current form as an alternative way of paying a mana cost. This affects how it interacts with, for example, Eldrazi Spawn tokens' sacrifice-based mana ability (mana abilities must be activated before paying costs), and effects such as Trinisphere that care about total costs. Guilds of Ravnica introduced cards that care about the number of creatures that convoked it.

Rulings

 * You can tap only untapped creatures you control to pay for a spell with convoke that you cast.
 * Creatures tapped by convoke cannot be used to attack
 * You can use creatures with summoning sickness to pay for Convoke costs.
 * Convoke doesn’t change a spell’s mana cost or converted mana cost. For example, the converted mana cost of Endless Obedience is 6 even if you tapped six creatures to cast it.
 * 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.
 * Because convoke isn’t an alternative cost, it can be used in conjunction with alternative costs.
 * 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.
 * If a creature you control has a mana ability with undefined 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.

Cards that grant Convoke

 * Chief Engineer (Artifact Spells)

Cards that care about the number of creatures that convoked it

 * Venerated Loxodon