Annihilator is a triggered ability that triggers when the creature with Annihilator attacks. When this triggered ability resolves the defending player chooses and sacrifices permanents equal to the number after the Annihilator keyword ability.

What does Annihilator do in MTG?

702.86a Annihilator is a triggered ability. “Annihilator N” means “Whenever this creature attacks, defending player sacrifices N permanents.”

Does annihilator trigger before combat?

Yes. Annihilator is a triggered ability, that triggers when the player declares the creature as an attacker. Removing the source does not remove the ability from the stack.