pf2e.fyi
From Pathfinder Impossible Magic, which isn't enabled.

Undying Resilience

CLASS FEATURE 17

Your body has been necromantically enhanced. Your proficiency rank for Fortitude saves increases to legendary. When you roll a critical failure on a Fortitude save, you get a failure instead. When you fail a Fortitude save against a damaging effect, you take half damage.

Pathfinder Impossible MagicORC