The Twilight Sphere
Massive Damage Threshold
If you sustain damage from a single attack that exceeds your Massive Damage Threshold, you must roll a Fortitude save (DC 15). If this saving throw fails, you immediately drop to -1 hit points and begin dying, regardless of your previous hit point total. You Massive Damage Threshold is equal to your Constitution Score plus your Armor, Shield, and Natural Armor bonus.