If a creature with annihilator attacks, when does the defending player sacrifice permanents?

Annihilator is a triggered ability that triggers when the creature with annihilator attacks. Specifically, per CR 702.86a, the ability triggers at the moment the creature is declared as an attacker, but it goes on the stack and resolves at the next opportunity — which is the beginning of the declare blockers step, before blockers are even declared.

Here is the sequence: during the declare attackers step, once all attackers are declared (CR 508.1), triggered abilities that triggered during that step are put on the stack (CR 509.2 governs the transition to the declare blockers step). The annihilator ability then resolves before the defending player declares any blockers, forcing the sacrifice immediately.

The defending player chooses which permanents to sacrifice when the ability resolves. They must sacrifice exactly X permanents, where X is the annihilator number (e.g., annihilator 2 means sacrifice 2 permanents). If they control fewer permanents than X, they sacrifice all of them (CR 702.86a).

Example: Your opponent attacks with Emrakul, the Aeons Torn (annihilator 6). The annihilator 6 ability triggers and goes on the stack. Before you can declare blockers, the ability resolves and you must sacrifice 6 permanents — potentially including creatures you would have used to block Emrakul.

This timing is crucial: because the sacrifices happen before blockers are declared, the defending player may lose the very creatures they intended to use as blockers, making annihilator especially devastating.