Annihilator

Annihilator is a triggered ability introduced in Rise of the Eldrazi that appears on all the colorless Eldrazi creatures from that set. According to Mark Rosewater, it is unlikely to be reused.

Description
Annihilator only appears on colorless Eldrazi creatures, and on a single colorless Eldrazi Aura, Eldrazi Conscription. The mechanic serves two purposes: it helps break the large board stalls that can occur in Rise of the Eldrazi games, and it demonstrates the storyline idea of the Eldrazi as extremely powerful destroyers of planes in an in-game mechanic.

Rulings

 * The defending player is the player that the creature with annihilator is attacking, or the controller of the planeswalker that the creature with annihilator is attacking.
 * Annihilator abilities trigger and resolve during the declare attackers step. The defending player chooses and sacrifices the required number of permanents before they declare blockers. Any creatures sacrificed this way won't be able to block.
 * If a creature with annihilator is attacking a planeswalker, and the defending player chooses to sacrifice that planeswalker, the attacking creature continues to attack. It may be blocked. If it isn't blocked, it simply won't deal combat damage to anything.
 * In a Two-Headed Giant game, the controller of an attacking creature with annihilator chooses which of the defending players is affected by the ability. Only that player sacrifices permanents. The choice is made as the ability is put on the stack. See rules 810.7 and 805.10e