Ulamog's Crusher

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.

Wotc — December 07, 2018

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.

Wotc — June 15, 2010

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.

Wotc — June 15, 2010