Covenant of Blood

If the target permanent or player is an illegal target when Covenant of Blood tries to resolve, it won't resolve and none of its effects will happen. You won't gain any life. (Any creatures you tapped to cast Covenant of Blood remain tapped.)

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. Convoke doesn't change a spell's mana cost or mana value.

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