Pathrazer of Ulamog
Creature — Eldrazi
Annihilator 3 (Whenever this creature attacks, defending player sacrifices three permanents of their choice.)
This creature can't be blocked except by three or more creatures.
No thought but hunger. No strategy but destruction.
Illustrated by
Rise of the Eldrazi#9UncommonEnglishLegality
Rulings
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.