Bearer of Silence

Devoid (This card has no color.)
When you cast this spell, you may pay {1}{C}. If you do, target opponent sacrifices a creature. ({C} represents colorless mana.)
Flying
This creature can't block.

Oath of the Gatewatch

Devoid (This card has no color.)
When you cast Bearer of Silence, you may pay {1}{C}. If you do, target opponent sacrifices a creature. ({C} represents colorless mana.)
Flying
Bearer of Silence can't block.

Oath of the Gatewatch Promos

Bearer of Silence's triggered ability targets only the player, not any creature. For example, the player may sacrifice a creature with hexproof.

Players can respond to the triggered ability, but once it starts resolving and you decide whether to pay {1}{C}, it's too late for anyone to respond.