Ulamog's Crusher

• WWizards of the Coast2010-06-15
  Official Ruling
  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.
• WWizards of the Coast2010-06-15
  Official Ruling
  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.
• WWizards of the Coast2018-12-07
  Official Ruling
  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.

Rulings via Scryfall