Any critical hit to the zombie reduces it to 0 hit points instantly.
Thank you! I've always found it bizarre in prior DnD editions zombies zombies are immune to both critical hits and sneak attack damage. After all, these guys are vulnerable to head shots or even un-killable without them. I already knew that they were dropping those rules for undead in general, which is good. But I've got to say I'm especially pleased that they went the extra step and made zombies extra vulnerable. This doesn't seem to apply to higher level zombies which is fine, they've got a different set of thematically appropriate properties.