pf2e.fyi
From Pathfinder Lost Omens Cheliax, Infernal Inheritance, which isn't enabled.

Hardening Necrosis

FEAT 9
BUGBEAR
Activity reaction
Prerequisites koblak bugbear heritage
Frequency once per hour
Trigger You would take a critical hit that deals physical damage.

Your partly undead flesh becomes resistant to the strongest physical attacks. You gain resistance 6 to the triggering damage; this resistance increases to 8 at 13th level, and to 10 at 17th level. However, you are Slowed 1 for 1 round as your joints seize up and your hardened flesh makes it difficult to move.

Pathfinder Lost Omens Cheliax, Infernal InheritanceORC