pf2e.fyi
From Pathfinder Book of the Dead, which isn't enabled.

Unkillable

FEAT 8
ARCHETYPE
Activity reaction
Archetype Zombie
Prerequisites Zombie Dedication
Frequency once per hour
Trigger You would be reduced to 0 HP.

You're relentless and nigh unkillable, shrugging off blows that should destroy you, though they might make your body start to fall apart. You are reduced to 1 HP instead of 0 HP. You must succeed at a DC 6 flat check or your body deteriorates.

Pathfinder Book of the DeadOGL