"Whatever the Eldrazi's purpose is, it has nothing to do with something so insignificant as us."
—Nirthu, lone missionary

「エルドラージの目的が何であれ、我等の如き取るに足らないものとは何の関係も無いことだ。」――孤独な宣教師、ニルトゥ

ウラモグの破壊者

滅殺２（このクリーチャーが攻撃するたび、防御プレイヤーはパーマネントを２つ生け贄に捧げる。）毎ターン、ウラモグの破壊者は可能なら攻撃する。

Ulamog's Crusher

Annihilator 2 (Whenever this creature attacks, defending player sacrifices two permanents of their choice.)
This creature attacks each combat if able.

Modern Horizons 3 Commander

Annihilator 2 (Whenever this creature attacks, defending player sacrifices two permanents.)
Ulamog's Crusher attacks each combat if able.

Commander Masters

"Ulamog is dead, and his brood is dying. Some of them just don't know it yet."
—Nirthu, lone missionary

Ultimate Masters

Duel Decks: Zendikar vs. Eldrazi

Annihilator 2 (Whenever this creature attacks, defending player sacrifices two permanents.)
Ulamog's Crusher attacks each turn if able.

Modern Masters 2015

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.

If Ulamog's Crusher can't attack for any reason (such as becoming tapped or having just entered the battlefield), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.