Needs review
Source: Kalamar's Player Guide

Description: You are unlikely to die from a single attack.

Prerequisites: Undying, CON 13

Your character does not need to make a Fortitude save to resist death from massive damage.

Normal

You must make a Fortitude save at DC 15 to avoid instant death when you take 50 damage (if Medium-sized) or 40 points of damage from a single attack.