Chord of Calling

Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.

If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before 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.

When using convoke to cast a spell with {X} 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 {X}{G}{G}{G}) and choose X to be 3, the total cost is {3}{G}{G}{G}. If you tap two green creatures and two red creatures, you'll have to pay {1}{G}.

If a card in a player's library has {X} in its mana cost, X is considered to be 0.