MASSIVE DAMAGE
Any time a character takes damage from a single hit that exceeds the character’s massive damage threshold, that damage is considered massive damage. A character’s massive damage threshold is equal to the character’s current Constitution score; it can be increased by taking the Toughness feat (which grants a +3 bonus to massive damage treshold instead of hit points) or by wearing armor. Light armor increases the massive damage treshold by +0, medium armor by +2 and heavy armor by +4.
When a character takes massive damage that doesn’t reduce his or her hit points to 0 or lower, the character must make a Fortitude save (DC 15). If the character fails the save, the character’s hit point total is immediately reduced to –1. If the save succeeds, the character suffers no ill effect beyond the loss of hit points. Creatures immune to critical hits are also immune to the effects of massive damage.